It's so interesting, because my impression was roughly the opposite.

Once you get the muscle memory for ninjutsu, NIN is quite easy (not at all in a bad way). I had the impression that MNK was the bigger-brain class—MNK has 50 difference openers and 45 subtly difference rotations to optimize various things. NIN has the one opener and the one rotation.

NIN is not really punished at all in savage for bosses going invuln and whatnot. Your rotation is almost completely flexible outside of burst, and for any savage fight, there will be norms about when it makes sense to hold burst until after an invuln window or not (P2S before limit cut, P3S adds, etc.) The one buff you have to maintain on NIN lasts 60 seconds and can be reapplied in 1 GCD after an awkward invuln break.

Maybe I just need to actually try MNK, haha! I basically just play NIN most of the time I'm on.