Phantom’s usefulness is directly related to whats going on around you. The only time when he’s doing any good is when the enemy isn’t paying attention. If you directly compare him to the Spy in TF2, of course hes going to fail. His cloak/decloak is clearly audible, and he’s pretty visible when you’re on your toes.
However, he’s a lot better when his target is already in the fray. He’s less of a spy, and more of a DnD rouge. Flanking and ambushing are what he does best, not backstabing. You can try the backstabby method, and it might work when someone is rushing to clutch an objective, but more often then not they’re just going to turn around plug you.
Which is why he has both the shielding and the 110 HP. That audible decloak is MUCH harder to hear over gunfire and artillery. When a player is already focused all their attention on trying to track an enemy, he’s going to be much harder to see. Phantom thrives in chaos more than a Spy ever could. Run in on a vulnerable target/targets, spray down with KEK or Crotzni, run off and hopefully draw out the enemy with you.
These are just my observations of what makes a good Phantom, and a dead one. I mention Spy because its the comparison most players are going to make. When I first tried him, I tried to play Spy. I was a very, very dead Phantom.