You're not alone, the thing is I really wanted to enjoy this with the first Black Ops, it was right up my alley (zombies, arcade style fps) but after 10 minutes I figured out the ennemies HP scale the more rounds you clear, I hate this type of design,they could've include more ennemy types, or at least use color swap but no.
It ruined the consistency in the gameplay, do a zombie need 2,3 ...10 shots? By the time you figured it out the game change the rules, now it's 15! There's a reason other games that uses scaling often display life bars or damage dealt with numbers.
But wait ! This is supposed to be a scary experience, we can't have life bars or flashy numbers flying everywhere that would be less immersive!
So what? Is it a problem if everything is just less predictable, if it's not "precise" enough for you? YES it matter a lot in this situation, you got no room for error there, the levels are cramped, everything is narrow, obstacles everywhere, you can't avoid encounters for too long the zombies will overwhelm you, it's a survival game you got 1 chance and they hit hard and fast, 2 or 3 hits and you're dead, most of the time you can't even react quickly enough to avoid the fatal second hit, add cryptic/hidden requirements to progress and use basic mechanics like mandatory upgrades for your guns...kudos for the fans of this mode you have my respect.