It is not overpowered. People automatically associate overpoweredness with high damage, and that is far from the case.
The Assault Class is a powerful class with high health, but it has its drawbacks. Test out the class, and you will find it is not quite as easy as it would seem.
I personally suck with the Assault ... a lot :P I find the Assault is best suited when enemies are directly in front of it. When jumping and weaving starts up though, it is in trouble, which is why a good, quick Huggable can outclass them. They don't have the consistant damage output like the Heavy does with its automatic weapons, which makes its slow speed even more troublesome. Yes, they do have massive damage, but if they can't hit you there's nothing to worry about.Â
My best assault score is 12 kills 3 deaths. But i get this spazzing out glitch that makes my camera start moving when i don't want it to, then i accidently fire, and say bye-bye to my life.
I feel that the only reason the Assault has more health than the Heavy is because some of it is almost always blasted away by the classes own weapon during combat.Â
There is no best class; it all depends on your play style. For me, I would say that the Soldier is the best class, but for you, you think it's the Assassin.
Actually I've only ever had problems with Bearail Guns. Yes, I can kill them, but they can take so much health with the first shot. It can cripple a Soldier's health.
I own the Assault, and it takes quite some practice to actually be good at it. The Assault's biggest weakness is EXTREMELY close range. You will just end up hitting yourself most of the time. His melees are also one of the weakest in the game.
If the melee were powerful, then it would be too overpowered. Personally, I didn't really need practice with it; I got the hang of it when I first used it.