When looking at some structures I noticed that even for trees things like buttons, fences and diorite (stairs) were used

In my opinion you shouldn't use elements like that for nature - you can decorate a house but a tree should stay a tree even if adding those elements might give it a better shape (see mss:birch)

One of the "worst" examples in my opinion is the waterfall-splash, it just doesn't look / feel right:

I'd say the same applies to crafted or processed blocks (e.g. minecraft:deepslate_tiles in mss:jungle) - why would a natural occurence have these manually created blocks (on its lower side / bottom)?

You could say it might be due to how it got up in the air in the first place (e.g. it was part of a structure which then got separated) but the rest of the floating island doesn't really give information to support that theory