Phantom has been out for more than 24 hours. Since the very beginning some players assured he was OP. My opinion is that most players does not understand how to play Phantom and thus how to play against him.
A first glimpse on why Phantom seems OP
While most players did not unlock Phantom we have seen a lot of messages stating that Phantom is OP based on what these players have seen in-game even if they did not play Phantom. And this leads us to a bias called confirmation bias which is the tendency to point out information that confirm our hypotheses/beliefs while minimizing the importance of (or even ignoring) information that refute our hypotheses/beliefs. I will give two quicks examples.
I am pretty sure you have seen someone arguing Phantom is too strong based on the score of one player like on this screenshot So many points, so many kills, the merc must be the reason. Have you seen the other Phantom on the scoreboard? Since Phantom is OP, this player must be terrible. Or is he? Most people will only notice the Phantom with a high score which is not relevant.
You have also probably read someone basing his opinion on the kill feed: 4 melee kills in a row from Phantom? OP OP OP OP OP. Surely it cannot be a misplay form the other team, this never happens. Oh wait (yes I should have killed the Fletcher 
)
Another explanation
Phantom is not so easy to play. No he is not. Just like a spy in TF2, when you do not know what it requires to be a huge pain in the *** it might look easy. When you are being stomped you are tempted to lay responsibility on imbalance. Do not lie, we have all done it. You are being killed again and again by this player who is always using this class you have never really tried. The class is probably broken. It must be that. And one day you give a try to this very class and you fail miserably ending last of your team with your worst ratio ever. You will quickly forget this incident and go back to your previous stance: the class is OP. Or you will persevere and you will quickly understand you need to play in a totally different way.
This is why we mostly find extreme opinions on such classes. Once ingame, players that give a try to Phantom but do not manage to understand how to play it will probably quickly go back to a familiar merc. Those who are comfortable with it have already experienced a similar gameplay and will end up at the top of the scoreboard. Not because the merc is OP but because they are good players overall.
So what is the point in all that? Just to show that you should not rely on scoreboard, killfeed, effectiveness of a couple of players or feedback based on a few minutes of gameplay. This is why I made this post, not only to give some feedback but also to offer a more detailed view of this peculiar merc and maybe help players understand this merc better.
Phantom
I suppose you have already seen his role call video (if not here it is). I will start with three misconceptions I have read here and there regarding his ability and weapon.
Phantom can turned fully invisible => No. You can still see him, he looks like some kind of black smoke. Human-shaped. With a Katana 
You just need a few games to get used to it.
Phantom is invincible while his ability is active => No. You can consider his shield like a second lifebar which is active only when invisible. If you shoot him while he is invisible it will depleted this second lifebar and then his normal lifebar. Since a powerful blow with a katana is enough to empty a full bar, I would say it roughly grants the equivalent of 90hp. Keep in mind that this amount decreases a LOT when moving: Phantom does not always have +90hp when invisible.
Phantom can instagib players => No. If it happens it is most likely a bug just like Sparks’s ability to instagib with her railgun when she was added to the game. The katana behaves like other melee weapons. When your hp reach 0 there is a 366ms period before you can be gibbed and you cannot be hit twice by the same blow of a melee weapon. Sooo no instagib.
Regarding speed, Phantom’s movement speed is 410 (see other mercs here) and most importantly, unlike other melee weapons, the Katana seems to slow you down quite a bit. Here is a short clip showing that with a Katana it is not that easy to catch up with a Skyhammer using his assault rifle. I think there is almost no movement speed difference between the KEK and the Katana.
Why bothering you with this ? Because it means almost any merc using his primary weapon can back up while shooting at Phantom. If he was not yet at melee range he will probably not be able to hit you with his Katana. In order to hit you is best option is to run at you in a straigth line but in this situation it is easy to score a lot of headshots and if you aim carefully he will go down quickly. This is not specific to Phantom, when someone is rushing you with a melee weapon or shotgun you are in the same situation. Nothing really new here.
More on his ability.
[INDENT]When active, a sound will be emitted for everyone to hear. You should really pay attention to sound, it helps with proximity mines, heartbeat sensors, footsteps and now cloaked Phantom.
In my opinion this, along with the partial cloaking, are the reasons why Phantom has been granted a shield. In a week, most players will manage to notice cloaked Phantoms (and if not they will have 2 weeks of free rotation to learn painfully!). Without the extra shield Phantom would probably become completely useless after a few weeks. Remember the shield is not that powerful, is directly linked to your invisibility and is deactivated as soon as you use a weapon. Thus you cannot force your way in, tanking damage. You will not absorbed enough damage and it will also removes your invisibility preventing you from hiding.
To be effective you have to sneak around the enemy team. You could already do that with Aura or Proxy thanks to their high movement speed. Phantom is much slower and his shield allows him to survive what would be stupid deaths for someone trying to sneak in. I am talking about mines, airstrikes, grenades or simply someone shooting you. With Aura and Proxy you can avoid losing all your hp especially with the long jump but Phantom is not quick enough and he should not be faster than he is now, if you have read the previous paragraph you should guess why. Thus he has a shield. Small enough so he is not super tanky but sized to be useful.
The Katana
Too much damage, too quick, too much range, this is what you can read on the katana. Let’s see how it works. Like any melee weapon you have two different attacks: a powerful blow (right click) and a quick slash (left click). The powerful blow deals 90 damage and his animation lasts 1 second. On the other hand the quick slash deals 60 damage for an animation of 0.77 second. Chaining a powerful blow with a quick slash allows you to kill any merc up to (and including) Fragger. Does it seem too much? Let’s have a look to the Stilnotto: 85 damage / 1 second and 35 damage / 0.5 second. And now the cricket bat: 80 / 0.9 and 55 / 0.77. Ow. So with those weapons you can kill the same mercs with the same combo except for Fragger and it will be quicker than with the Katana. Note that before the last patch the Stillnotto used to deal 90 damage (kill Kira, Aura, Proxy and Sparks in one hit). What it means is that even before Phantom was added, you could have played the same style — heavily focused on melee — and be almost as deadly.
Tha Katana simply looks like a slightly buffed Cricket Bat. I am pretty sure the range is a bit better but I do not have any value. For a class dedicated to melee combat, I think it is fair enough. Still there are two issues and they can stack. The first one is the Chopper augment that increases melee damage by 20%. It can be associated to the Cricket Bat for Arty, Bushwacker, Fletcher and Sparks. I think it is OK since none of these mercs is very good at sneaking in except maybe for Sparks but she has very low HP. On Phantom you can have a buffed Cricket Bat (aka Katana) AND Chopper. This might be too much.
The second issue is related to the powerful blow of the Katana. You can hit multiple players with it, just like other melee weapons, but the Katana has a unique property: the blade deals damage for a longer period of time. When you perform such an attack, during the first part of the animation you will raise your weapon and then strike. You can deal damage only from this moment. The weapon will then stay in that position for a split second before returning to its idle state. The active state of the cricket bat is roughly 0.2 second. The active state of the Katana is 0.6 second. And this is why it is possible to create such wonder I really think this should be reduced, maybe to 0.3. The powerful blow should remain an attack requiring a bit of accuracy, just like the other melee weapons. Having a weapon a little bit better than the Cricket Bat one very point is fine I guess but this is too much.[/INDENT]
How to counter him
I will not repeat myself too much but the most important thing is to look for cloaked Phantom just like you look for mines, snipers or sticky bombs. You will get used to it. Shooting him while he is cloaked IS useful since it drains his energy. Pay attention to sound and communicate. When an enemy is behind you, be it Phantom or any other merc, you should tell your teammates. If you are playing Vassili, do not forget that your heartbeat sensor prevents Phantom from using his ability!
Now if you are facing a good Phantom, think like a Phantom (or a spy from TF2) and not like another merc. Just like in TF2 if you are good with Phantom you should be good at chasing him and vice versa. Look for hidding places (where would YOU hide?), watch above your head, think of the trickjumps he might used (learn them !) and check your back if you know a Phantom is around.
What if you encounter a Phantom coming for you? Shoot him while going backwards. A turret can provide a very good support since Phantom will most likely be using his Katana.
Role
Some may say Phantom is useless for his team. Think about the role of these mercs: Nader, Fragger, Vassili and Rhino. They are all made to kill and besides this main purpose they are used for secondary tasks. Nader can deny an area and is strong against static position. Rhino is good at holding a static position. Fragger can easily deal with stacked players and gib them with his nades. Finally Vassili is here to kill and gib the most important mercs of the other team: medic, engie, sniper or simply the best players. And now we have Phantom who is pretty close to Vassili. With both mercs you can kill the first player you see again and again, without gibbing him and be almost useless for your team. Or you can think a bit and pick your targets. For Vassili it can be hard to hit a player hiding such as Sparks, another sniper, an engi repairing an objective or anyone hiding near a healing station. This is where Phantom excels. You are behind the enemy, hidden and waiting for the best moment to strike.
Phantom is a good counter to Sparks, Aura (or anyone near a healing station), a busy engi and Vassili. He is also countered by Vassili, Rhino (a good Rhino I would say), Arty, Kira and any fast merc that have spotted him.
Why such a merc in Dirty Bomb?
This is entirely my opinion and not based on any statement from developers 
Dirty Bomb offers an interesting movement system, long TTK and the level design give you a bit of freedom. You can play in a classic way, using your firearms or you can take advantage of these three elements to play in a sneaky way. Like I said, even before Phantom was added you were able to go around your enemies and kill them using melee or a short range weapon. You can see a lot of very good examples of this kind of play in this video from Sagan on Roids Phantom is just a way to recognize this part of Dirty Bomb’s gameplay. Even if you took out Phantom from the game you would find a lot of Phantom-like players turning away from their current merc core gameplay. Isn’t Phantom the merc that fits the best the motto “Play Dirty”?
I really think it is too early to call Phantom OP. His gameplay somehow already existed in Dirty Bomb but it was not frequent to encounter a player that used it. Players need more time to adapt and play with it, especially during the free rotation to realize he is not that easy to play with. Sorry if there is any mistake or typo, I started this post at 1am and it is now 6am, I am a bit tired