I do love Blood, but I will admit the enemy roster is pretty much carried by the cultists, which are not only some of the best aspects of the combat, but also some of the worst. Best because they're the only enemies in the game that actually pose a threat to the player, chucking dynamite into your spot and in general keeping you on your toes. However once you start cranking the difficulty up from lightly broiled, the fun factor with them begins to slightly diminish.
Now, I don't have an issue with hitscanners in games, methods to make them feel fair to the player do exist such as giving them a wind up, or in the case of the ones with machine guns, fire in short bursts to allow more leeway for you to get behind cover. The HECU soldiers from Half-Life are a good example for these practices. The issue with the cultists on higher difficulties is that they have zero wind up, and start firing at you the moment they're alerted to your presence. Fanatics are the most egregious, since their tommy gun can effortlessly riddle away your health before you even have a moment to retreat. They can be stunned when they get hit, yes, but not much can save you from their instantaneous attacks aside from memorizing their locations and attacking pre-emptively.
As for the rest of the monsters, most of them boil down to either being trivial, or just flat out obnoxious. Gargoyles and Phantasm for example aren't difficult to deal with at all, but they have a habit of staying up in the air for a varied amount of time while constantly screaming at you. Butchers, as well as most enemies with projectiles, can barely, if not at all, hit you when you're crouching. They are still fun to fight with, just not that difficult compared to the cultists.
Despite all this, I still love the game. And while Duke 3D is probably my favorite build engine game overall, Blood does eclipse it with its atmosphere and weapon roster. But I think Duke 3D and Shadow Warrior's enemy roster is better than Blood's, but really they all have their own flaws.