As Balance goes, the Classes ARE balanced, sure you could say having a whole team of Engies makes for some work to be done, but the same could be said about having a whole team of Medics.
I've had games where I'm dutifully defending a spot and doing quite well with my wingmen, encountering 4 Medics and NOT BEING ABLE TO KILL THEM, one goes down, gets up, kill the Reviver and the Revive-ee and they both get insta-Revived by a third all the while the forth is hanging back and buffing everyone. Sure the Limit on Pips would stop them eventually, but that's still a lot of effort focused on only half the damn team ya'know?
But really, if it can be abused, people will abuse it, no amount of tweaking, nerfing, powering up etc is going to stop SOMEONE from finding something else to abuse. As it stands if a whole defending team is piling up Turrets like they are going out of fashion, a single EMP or Grenade will generally knock them offline anyway. Hey, if you're well trained in Operative-ism hacking a turret behind where the enemy is standing and then setting off a lucky EMP or even standard Grenade before you're murdered, will do the job in a jiffy.
There are solutions to all problems in situations like that, the list is near endless. It's what makes this game great for me, the need to tactically find victory through adversary.
You missed the point entirely what I meant by that. What I was saying was every defense mission "requires" an engineer to succeed, since it is the most direct objective oriented class. Medics can revive each other as much as they want, but if it is a team full of medics and the enemy manages to put a single charge on that pillar, it is game over. Engineers aren't chumps that can only defuse bombs, they also have weapons buffs and other stuff like mines and turrets. By going along with your logic I can say that engineers can simply place mines and turrets everywhere making it impossible for the enemy team to invade + they can buff their own weapons so that medic team of yours can never outshoot the engineers team. .,,, and + they are able to defuse the HE bomb if it manages to light up, oh how unlikely it may be. See? I can go off on that tangent as well. Abuse is abuse, but then there is smart abuse. There's a reason why you don't see your medic team spammed on every defense map, but yet you see a good amount of engineers being used in reality.... and even when there is not a spam of engineers, you'll hear someone on the mic complaining that the team doesn't have enough of them.
*Reminder I am still talking about defense here.*
I mean sure the Operative can knock out the turret and mines with his emp grenade... but then what? Dance around the charge site as the time ticks down?