In my opinion, all mercs are as fine as is, and I have racked up some decent kills with the (need adjustment) mercs. The only merc I would say needs adjustment is Sparks as her primaries are the weakest, but what Sparks uses there primary? EVERY to a majority of Sparks players use there revive-gun. So that being said Sparks is a hip-firing sniper. However she’s really squishy, with 80 HP… but she’s ties with Aura at 470 Speed so you can use her speed as an advantage. It’s all about how to play mercs, and this is a competitive shooting game, so each merc is going to require a different skill-level to play with different abilities. Other than that her revive gun requires a charge, so you might want to get a little spacing and positioning when using it, then firing it. As a hip-fire sniper she might not have the greatest aim. I think Sparks guns should be switched to Blishlok, Crotzini, and Hochfir. This I think makes her stand a more chance in a fire-fight instead of being easily squished at a low 80 HP. Med packs do come in handy though.
Sparks needs adjustment
Sparks is a very high skill merc, probably the highest. Which is why people tend to not play her as much, even though on base level she is very helpful to a team. I personally think she is ok, but I think it needs to be made a bit easier to get her full potential. Some slight changes (increase) to the range of both revive hp and damage would work wonders to her long range capabilities, and maybe increase the hitbox of the Revivr projectile so it will be easier to hit someone. Other than that she has very good survivability if you play her right, hence why good players tend to live a long time with her despite her low hp.
I was simply saying that she should have a change in primaries. She’s a hip-firing sniper but I think she’s easily squish able so maybe she should have a smg like Proxy? I mean, the Empire-9 does 9 dmg, MP400 11, and Tolen MP 9
Aim better
Her Revivr is her primary, her machine pistols and pistols are like dual secondaries for her.
She doesn’t need an SMG, not unless her packs are removed, and even then that may be debatablely op.
revivr does 130 dmg in head on full charge, her medpacks heal are healing instantly half of her hp
gut gut 
You should probably use much more the REVIVR. In the case you have missed it, it’s a weapon that also allows you to damage enemies. The REVIVR is actually supposed to be Sparks’s primary, explaining why she only gets machine pistols as her main weapon. The REVIVR is also insanely powerful : a fully charged shot deals 65 damage, that 130 damage if you aim the head. It can on hit kill most of the mercs in the roaster.
She’s also the best medic when it comes about self sustain. While her medpacks are not enough to sustain the team with healing, they’re enough to sustain herself and one other person. With just two medpacks dropped for yourself, you can climb back from the red zone back to full HP. This is just insane since you pretty much have to one shot a good Sparks to hopefully kill her. If you get killed too oftenly, you probably play too agressively.
Even worse : in competition where friendly fire is on, she can just ignore her main issue ! Sparks is supposed to focus on revives at the cost of her own healing efficiency, but friendly fire allows her to just kill a low hp merc just to revive him one second after. And that’s pretty much crazy.
Sparks is also one of the most common bans in competitions since the introduction of the ban system in DBN and other e-sport leagues.
Considering all of this, I wouldn’t ever consider a buff for Sparks.
Did you see how many Sparks are in one match? mostly there are only 1 or 2, or even 0. Sparks is great, she requires lots of skills to use, and she almost can die from anything, Burst Rifle headshots, explosives, etc. I got her as my first medic, and she will be one of the medics that i love to use, as @Eox said, the REVIVR is her main weapon, her weapons are only secondaries. I think giving her SMGs will only happen if the damage from the REVIVR gets deleted, with would never happen, because if that would happen, then Sparks would get destroyed or even killed. So basicly giving her SMGs would be pretty stupid-ish.
sparks is been underrated for her been more likely to revive and not heal and that is true since its impossible/really rare to find a high lvl not finish off and thats what make her unused in high tier but with newer player it can be a real pain to deal with since when she revive she give more health then the defib she more low tier then high tier (like teemo in lol he could be a pain for new players with mushrooms everywhere but useless in high tier )
so i guess making sparks more avaible in high tier for its preimarry objective revive/heal cause you can sometime see a really good sparks with fast headshoots so yeah i guess she need some adjustment
but i dissagree with you sparks is meant to be a sniper-medic and she is meant to use mostely her revivr with good aim it can deal a lot of dammage and with the machine-pistol to finish him she is more likely high aim = high reward in killing but she lacking in her primary objective reviving and healing (i perssonnaly use mostely the health to heal my self and run away or in a mid fight for a fast back up heal)
I’d definitely like to see some sparks changes – I like her a lot, but she seems like more effort than she is worth. She isn’t that much more effective at range than any other smg medic, and she is far harder to play.
Suggested changes:
Revivr dropoff at range: damage drops to 80%, revive health drops to 50%. Now, she will be significantly stronger offensively at long range (though she still can’t one shot vassili or redeye past 32m), but her cross map revives will be a lot weaker (65hp at full charge with get up).
Health/speed: 90 hp, 440 move speed. Essentially, give her kira’s chassis. She is a bit harder to instakill (and she can now survive a stuck sticky with unshakeable), but she can’t run away quite as quickly. When she goes into “run away while spamming medkits” mode, you have more time to finish her off.