I've see this recommended a lot, actually. Is losing Berserker really worth it? I've personally *never* taken a build with it. It has some great utility clearly, (does the 50% resistance give you less knockback vs sergeants?) especially versus arch-viles, but you lose the great perks that come with berserking. I'll keep an open mind, but could someone detail the benefits for me? Seems like in the cases where i'd appreciate the damage resistance i'd rather take a big hit and go berserk and then smack everything up.
The biggest problem with Berserk is the lack of flexibility, and how screwed you are if you end up in a bad spot. Berserk will never proc from the attacks of most enemies, and even some that may often have to roll in the higher ranges to cause you to go berserk. This problem is even bigger for Marines with their higher health pool (they need a 20 damage hit instead of 16 to go Berserk). Out of all the regular enemies only Barons, Revenants, Mancubuses and Arch-Viles are able to make you go berserk. Out of those the only reliable source is the Arch-Vile, who is guaranteed to make you go berserk unless you increase your maximum health above 60. All the others have an average damage below the threshhold, the baron being the worst case (they do 4-20 damage).
Arachnotrons are often among the worst for any berserk build. They hit hard, their damage can only be significantly reduced by already being berserk or having very specific armors (or Cerberus assemblies) and their attacks are among the hardest in the game to dodge. On top of that they are very fast. If you actually get into melee range with them they're complete pushovers, but in any other scenario they're one of your worst enemies.
The biggest problem is that Arch-Viles are your only guaranteed source of instant berserk, from any other enemy you could theoretically die before you even go berserk. Going berserk on your own usually requires several enemies within short distance of each other since few enemies can withstand enough damage to let you berserk on just one of them, and during that time you'll be taking the full brunt of any attacks aimed at you. Not to mention that once you actually go berserk it is often difficult to keep it running since you'll one-shot most enemies.
MB basically loses out on Acid and Plasma resistance aswell as the double damage and speed boost of being berserk. They also need to keep a knife in their secondary slot, but in exchange they get even more melee resistance than the berserk builds, they already get Dodgemaster as one of their requirements and the most important part - it's always active. MB faces a similar weakness that berserk builds do - they dislike Arachnotrons - but in my honest opinion I haven't played any build that enjoys fighting Arachnotrons unless it's on your own terms. Because they miss out on Acid resistance they're also worse at fighting multiple Barons, but they're better at fighting them in melee - in fact, I'd say MB is the best build at fighting in melee range. Even with basic armors few things will be able to do more than a few points of damage at best.
I often make an "early" P-modded Nanofiber red armor and use that until I find an armor and the mods necessary to make something better. It saves inventory space, at most I'll carry a fireproof red armor (or two) in case I run into multiple Mancubuses and the like.
If you would happen to find both an Onyx mod and an Energy-Shielded Vest on a MB run you might aswell just consider yourself immortal - even without a Nano mod to make it a Cybernano armor you'll still be taking minimal damage from anything that isn't rapid-fire or Barons.
An added benefit of MB is also the fact that if you happen to find a berserk pack you'll have the best of both worlds - much like the Energy-Shielded Vest case you might aswell consider yourself immortal while berserking.
To answer your question about knockback, damage resistance does not reduce knockback - you'll want either your boots, armor or traits to make up for that. That holds true for berserk aswell though.
I may be slightly biased towards MB for multiple reasons, but I've tried to be as objective as I can. My opinion is that the biggest strength of MB is the flexibility compared to berserk builds. Berserk builds often have the potential to do better, but they're at greater risk since they have downtime.
As for Vampyre; I'm currently doing an AoD/A100 N! run with a Vampyre build, and the short answer is basically what Sylph said. A longer answer is that you tend to play slightly differently - depending on your supplies and the layout you may play more cautiously than you normally would, but at the same time you always have the equivelent of access to infinite health orbs as long as you're able to set up a relatively safe position to spawnkill something.
