i believe that berserkers are NOT overpowered. A Support and Demo can easily waste a whole entire group of enemies and can easily kill FPs with aa + hunty spam or 2 pipes.Berserker can also kill a Flesh pound but it takes him longer.Supports can waste just about everything he can and can do it from a longer range and he can weld and unweld doors so fast,All his shotguns have major penetration And he can carry more grenades.In return the Berserker gets increased speed and damage reduction along with not being able to be grabbed by clots.Same with Demo they really cant do it at short range but at medium and long they can waste everything.They can also buy pipes for cheap and carry 4 times as much as any other class and can carry as much grenades as Support and get explosive Resistance.
I voted that they are even like the other classes
I think that you are missing the main reason why zerkers are OP. Let's go over your examples first.
First, let's make sure we are talking perk vs. perk. Not "This combination of supports + demo absolutely rule." Sure, combinations are powerful but it doesn't really match up when talking perks.
Yes, support and demo can kill the FP. However, it isn't quite as easy as you imply. I've been 1 shot killed by the FP while unloading a full drum into the fp (6 player HoE). Now support can toss grenades then shoot because the FP is susceptible to grenades. Demo, if he has room, can unload the m79 into the FP as well. Their big weakness is consumables. They can't keep doing it forever.
Put it this way. Say there was a 'nuke' perk. The nuke perk has a one shot kill every specimen within 50 yards. Can be fired twice. Now because he has the most powerful weapon of any perk, does that make him more powerful than the zerker? Nope. That nuke weapon while powerful is limited in the number of times it can be used. Same thing with demo and support. Sure, support can kill an FP by unloading on him with grenades and the aa12. But once he has to start kiting, that ammo is used rapidly. And that is the key.
Zerker's primary perked weapons never run out of ammo. The zerker is just as effective at the start of a wave as at the end no matter how often he has used his perked weapons. Add in that the zerker can fire his main perked weapons without pause the entire game. Then throw in the speed that allows the zerker to get out of just about any situation. No magnetic clot grab either. That speed also allows the zerker to create separation when he has to heal or load an unperked weapon. That speed also allows the zerker to shape the battle so that he fights what specimens he wants when he wants to fight them. Then put in damage resistances and permanent armor.
Pretty much any perk, except maybe flamer, can kill reasonably one FP. Now throw in 40 specimens and a couple of each of scrakes, sirens, and husks. Then add another one or two FPs. Now add another 40 specimens with assorted scrakes, sirens, and husks. Every perk is down to their 9mm now. Then throw in the FPs (and additional specimens) that spawn at the end of the wave. Every perk is running the entire map looking for spawned weapons and ammo (at slow speeds). Except the zerker. Zerker still has permanent armor and still has his full perked weapon damage.
That is why he is OP.