Ok well here is my opinion, although I do value yours, Fox is much harder to play tech skill wise then Sheik and Marth. I have no clue when Falco came in to this, but I digress. lol
Anyway, the way I see it, Marth's aerials are easier to land then Fox's simply because of the reach. Then again, one can argue tipping/spacing with Marth is just as hard as landing Fox's dair. Anyway, there is still more to Fox, obviously. Waveshining is one thing, but it takes a lot of work trying to get the "perfect" wave dash length or your waveshines, and use that length consistently.
Through my eyes, Fox has a far more complicated recovery system then both Marth and Sheik, possibly combined. Learning all the possible angles for Fire Fox recovery is insane. I swear I use angles that make my mouth drop.
So yeah, I'll refrain from rambling on an on, after all these are merely opinions.