I haven't played the Wii U version, so I can't talk about the Wii U stages a lot, only from what I've seen.
There's a difference between stage hazards like, say, Halberd's cannons and lift-off/landing sequences, and Magicant's Flying Man, or the Game & Wario stage.
The Wrecking Crew, Duck Hunt or Mario Galaxy stages look good to me, but Orbital Gate Assault looks absolutely awful to me, it's like PokeFloats, but actually terrible.
Maybe you and I have different expectations from Smash Bros., and its stages, but again: In a game where the goal is to beat the opposing player(s) - Smash is after all a fighting game - I feel like a stage that is so incredibly intrusive that the main goal on that stage it to beat the stage, not the other player(s) is completely destructive to that concept.
I can agree with that. Stage bosses, just like random rewards or very specific platforming or "minigames" within the stages are, in my opinion, very unpleasant.
Have you even READ what I said? Because if you did, you would realize that I, literally, said that I not only like "flat" stages, but also stages like Shadow Moses Island, Hanenbow - I specifically mentioned those because they're not "flat" or "featureless", and are in fact banned in tournament play. To bring up more examples, I also really like all of the F-Zero stages across the series, Delfino Plaza, Hyrule Castle from 64, the Pokemon Stadiums, all of the Metroid stages across the series (sans Pyrosphere), and many more. Some of those are tournament legal, some of them aren't, but I still like playing on them, and they're everything but featureless. I also don't mind scrolling stages like Rainbow Cruise or the 3DS Kirby's Dream Land stage, either.
There can be more than 2 extremes you know. There can be stages that fall in between crazy bullshit like making a 1v1 into a team fight, or having the stage suddenly flip controls for everybody (or worse, only one person), or the stage suddenly forcing you to stop all movement, etc., and "flat, featureless stages".
Planet Zebes/Brinstar is a perfect example of that. It's an interactive stage, the acid is rising and you can destroy parts of it, changing the landscape of the stage. Pirate Ship is another one - the stage remains unchanged, but various hazards appear, like the cannons in the background. Rainbow Cruise is a scrolling stage where you do have to platform, but the difference is that you actually also have time to hit people, and KO them (where in Mushroomy Kingdom, people will constantly hit the blocks at the top and generally are super hard to kill).
Aside from Spear Pillar (I really hate controller inversion and upside down flipping, in any game), I actually agree here. Pirate Ship and Frigate Orhpeon are also MY favorite stages in Brawl.