As a viewer, I would be bored if I had to watch and listen same song for hour or more. Like watching XIII's competetive matches, I don't know is there a stage that is most common pick, but I think Arashi no Saxophone 2 is the most common song you hear....
I think that stages in KOF in general have equal lenght, at least I haven't noticed. I kinda like interactive stages (stage hazards) in fighters (especially 3D-fighters), but hard to say do they work in 2D-fighters.
World Heroes had them, but never played enough to see were they good idea. There also was One Must Fall 2097 and I thought they were fantastic, especially the Desert stage. Imagine, huge robots fighting in a desert and sometimes fighter jets come and do strafing runs. That blew my mind when I was a kid.
Equal health... Never knew that was a thing, until I started following SFV. I guess that makes sense, but I don't know, characters already move and behave differently, so is that necessary?