The insane difficulty.
The berzerk pigcops or whatever they're called are essentially a one-hit kill. Sure, technically it's 3 hits, but they lunge at you before you can get a decent shot off, and even if you do manage to dodge, they hit you anyway. And their three hits are essentially SWINGSWINGSWING in the span of half a second with little chance of survival. Which means they're a one-hit kill for all intents and purposes.
Cover is essentially useless. Most of it is destroyed in one shot, and they hunt you down anyway, making hiding behind cover nearly pointless.
And of course, the fucking load times.