I think it's a personal preference thing. I get enjoying that there's loads to do and see now, but for me when the design got to visually busy and crowded, it made the levels harder to really get into. It's sort of what makes the second half of 2 lower quality in many ways. The levels are visually busy and sort of crammed with "stuff" in every corner just to fill space, and that's why they go downhill IMO.
Mystic Cave gets a pass because of the music, but I agree that the quality has some problems post-caves. That said, I still sort of stand by Oil Ocean as a fun zone with a good theme and visual design.
That infinite death pit in Mystic Cave can go do one though. Honestly, the best version is the version where Christian Whitehead turned that into an entrance into Hidden Palace.
Stunning excecution. Turning one of the biggest "fuck yous" in the game into a previously cut zone was genius.