Berserkers are pretty useless, and I've been playing one for a bit. To survive on HARD, you actually have to dance around like a monkey and hope to god you don't have to handle more than 3 enemies at once or else you are dead.
When a scrake comes, you have to parry, stun him with the hammer, and hope to god people do enough damage to kill him or else you die even with medics healing you.
The 5% damage reduction might as well not be there. Imagine a scrake attacks you for 30hp a swing (about a rough estimate without armor, they probably do more). Of that swing, you negate 1.5 damage. It takes about 4 swings to kill you, whether you have that 5% damage reduction or not.
25% melee damage? Pretty useless. Compare the number of swings other classes take to kill the same mob.
Siren resistance is lackluster since they aren't that much of a threat
Boat resistance is pretty lacking, even a medic which doesn't have to have contact with a bloats bile to kill them has a scaling bile resistance. A zerk, however, will have a very hard time not getting hit by bile whether by attacking a bloat or accidentally (from teammates shooting nearby bloats).
Movement speed is nice.
attack speed is the best of the bunch.
So in passive stats the zerk is really not that great, with only 2 stats that really end up making a difference, and even these do not provide the necessary defence
Moving onto perks...
Bulk ammo might be somewhat useful if you use the nailer, won't make much of a difference with the eviscerator due to it's low total ammo count.
Night vision is pretty lackluster, with a low use time, blurry vision, and not much brighter vision
Frenzy is moot, medics will heal you pretty fast so you're likely never low on HP. With your low total health, in a situation where you would loose enough health to make a difference, you would die.
Fake out is great, helps a lot with mobillity and fighting melee
Furious Defender - See frenzy, same logic applies.
Formidable is nice, especially against big zeds. You first attack will ensure they do less 15% damage if you fail a parry. Can be nice on large groups of gorefasts.
Take cover is very situational. Might get you of trouble if **** hits the fan, but let's face it, if your hp drops below 20 you're most likely dead whether you run faster or not.
Indimidate, well... 10% of light attack? not really that much. 15% of heavy attacks? With how much time they take to execute, not really consistent, more like a lucky strike.
Zed Time perks are more or less OK.
So yeah, a zerks perks are also pretty useless for most melee combat.
Zerk needs a buff.