Out of curiosity, what parts about Nocturne do you feel are too complex? Because to me Nocturne seems like a relatively simple set - there's no landmarks, projects, tokens, etc., and only one thing (Lost in the Woods) that's equivalent to Renaissance's Artifacts
The most complex single element is the Hexes. I put some work into having a simple set of Boons, to make sure they didn't bog the game down, and then that all went out the window with the Hexes. They were attacks, they couldn't be simple. Instead of "oh so I shouldn't do them," there they are. We had lots of fun playtesting them; then at the release event I saw casual players interacting with them and it's just, every Hex shuts down the game while people come to terms with it.
As I have pointed out many times now, to fully understand what Vampire does, you have to read 18 cards. That's ludicrous, that can't be right. Vampire, Bat. Twelve hexes. Envious, Deluded, Miserable, Twice Miserable. Even Tournament, which is record-setting complex, with a 2x2 grid of results to understand, still has only 5 more cards to read.
The set has too much going on in it; today I would do it as two expansions. One would be called Nocturne and would have Night, Boons, and Spirits; the other would have Heirlooms and some other new thing to go with them. Probably only 5 cards would make Boons, dodging less-fun situations e.g. "Idol could get you a Boon that actually hurts you." And well those attacks would need to do something other than hand out Hexes.
The Spirits and other extra cards aren't a problem by themselves; I do think it's better if an extra card is either used by one card or by several. If it's one card you can just put it away with that card, e.g. Madman with Hermit, and get them both out at once. If it's a bunch of cards, like the Tavern mat, you can get that out and then have it the whole evening. When it's just used by Cemetery and Exorcist, you have to get it out special for that game.
Adding e.g. Landmarks doesn't compare to that stuff. You can just not put out a Landmark if you want, if this game has a new player or you're new to the set or whatever; bam, a simpler game. The basic concept is simple and most of the cards are straightforward. I'm not aware of any issues people had with Landmarks. The tokens aren't tricky either; they're fiddly, you have to go dig out the tokens. I am not thrilled with the variety of tokens in Adventures, because you know, oh this game we need Plan tokens, and you have to search through the pile of tokens for them. Fewer tokens, more use out of each, would have been better. But Coffers and Villagers and VP tokens do just that, the sets get lots of use out of them, you just put them on the table for the evening.
Nocturne was great to playtest; for a sufficiently experienced player, it's full of great stuff. But for sure it's too complex.