For me there is almost no difference between UV and N! (as long as you use doors and liquids). Moreover, in some aspects N! is easier than UV, which is ridiculous IMHO and should be changed.
Personally, I find N! considerably more difficult than UV. A vast majority of my games are played at N! difficulty, and I find that on the odd occasion I play UV, it's a lot more relaxing, not having to worry if any of the monsters are going to get back up and being able to re-enter explored territory, confident that there's no danger. And yeah, if you use doors and liquids, etc. to get rid of bodies, it's a big help for N!, but those aren't always available, and sometimes the layout of the terrain doesn't make it easy to lure the larger monsters into body disposal positions. Say what you will about the difficulty, but I find it way, *way* easier to win at UV than N!
It doesn't help that the game's formula for deciding when monster's revive is pretty wonky. Sometimes, they get back up in the same second you killed them, so you can't even tell that they died, and sometimes, you can go for several minutes, with no revival happening at all. Also, sometimes, large groups revive simultaneously, regardless of what order you killed them in. (It's worst in the Mortuary, when you can get like 50 or more reviving at the same time and you're suddenly, instantly surrounded by some of the meanest monsters in the game) If anything "should" be changed about Nightmare, I'd say it's the revival formula, but I imagine that it would be a big headache for the designers to rewrite that. I could imagine a system where each corpse has its own timer to base its revival on, but then again, that might make the reviving a little *too* predictable and make things ridiculously easy. I don't know if there's an ideal solution for this, but it would be nice if I could at least trust the Bruiser or Shambler to stay down long enough for the two seconds it takes me to pull out my Rocket Launcher to finish them off.