ONML is a LOT worse than original Lemmings for what it's worth:
Crazy 2: Not terrible, but still rather hard to execute for such an early "serious" level in the game.
Crazy 7: Near pixel precision this early in the game? This is really freaking hard for its position, though not as bad as...
Crazy 10: A HAVOC CALIBRE LEVEL. In the 2nd tier. This is harder than all levels in Wild, All Wicked levels but Wicked 10, and at least half the Havoc levels. When I solved this I felt so amazingly well, except all the later levels in the set were so ridiculously underwhelming are beating this. It's a very well-made puzzle, but I mean come on middle of the 2nd tier out of 5? Really?
Crazy 11-20: LOL at all of these after beating Crazy 10 >_<
Wild 3: So I got to Wicked 10 and Havoc 10 before beating this. Really. The precision required to do this stage is really deceptive. And this is hand enough to place the skills to be at least Wicked.
Wild 10: Such a horrifically lazy level. Also easier than everything in Crazy except maybe Crazy 11.
Wild 11: Oh and then after that we get the hardest level in the set. Again, I find this harder to pass than most of the Havoc set. Ridiculous builder precision for this early in the game and there's a lot you have to think about to pass it.
Wild 17: uhhhhhhh if you know ANY way to stop a crowd without blockers (or you know how to free a blocker) this is a total joke.
Wicked 1: I like being able to pass the first level of a set without it being harder than most of the set, yea. Got that, Taxing/Mayhem from Lemmings 1? Guess not.
Wicked 9: Huh, this is easy just due to complete lack of any other solutions but the right one. Crazy 2 is harder.
Wicked 10: Hey this belongs in second half of Havoc too. I had so much difficulty passing this level as a kid. Again, a great puzzle ruined by being too early in the game.
Wicked 12-14: Ok these are all really freaking easy, especially 14. Seriously, you except these to be any challenge at all after 10 (and, to an extent, 11)?
Havoc 3: There's about 6,000 different ways to do this. Based on the title, it seems you had one specific answer in mind. Huh. Very easy, demote to Crazy.
Havoc 4: What's with all these really freaking hard levels right after or before really freaking easy levels? This is one of the 5 hardest levels in the game -- unforgiving multitasking, a very short timer, and a completely not obvious solution. Yea, much this back plx
Havoc 6: easyeasyeasyeasyeasyeasyeasyeasyeasyeasyeasyeasy
Havoc 9: One of my favorite levels in the game, but still very easy for its rating.
Havoc 13: ...
At least all the later stages are at least challenging (maybe not late Havoc, but still fairly hard) after 13.