I could never get far in Oblivion without the urge to turn it off and play Morrowind or Skyrim instead. But from what little I have played of Oblivion compared to the little I have played of Nehrim, Nehrim wins hands down. Escaping the mine was actually hard, but not in a boring or tedious way. Rather in a way that forces you to think, much like some older puzzle games (they don't make em like that any more).
Oblivion, by comparison, was kind of monotonous. Kill troll, kill zombie, kill rat, kill more rats, kill troll... ooh a potion... and so on. There wasn't much else to actually do while you were escaping the dungeon except maybe spam your magic to level it up. And every room seemed almost the same as the one before. I found the voice acting kind of flat too.
But in Nehrim you're introduced to a story (you're not just some random prisoner with no background at all) you have an explained reason for being where you are, each part of the mine you visit has its own unique feel, there are some basic puzzles (but hard enough to catch you out first time round) to further break up the monotony, the flora is so much prettier, there is a boss fight/puzzle, just overall it seems richer and like more love has gone into it.
Can't comment much on the voice acting since it's hard to tell if voice acting is good in an unfamiliar language, but the person who does the voice for the guy who cries for you to take you with him seems far more genuine than the voice actor of the guard who just watched Uriel Septim die. So, I'd still say the voices are better in Nehrim.