Peach IS very difficult, I think Pit is too, as far as Arrow Raining and such.
if the pit main is using arrow gimmicks like looping/arrow raining then he's doing it wrong
this is coming from a pit main with plenty of experiance of trying to "master" said "techniques" who was at one time considered the 2nd best at the gimmick only behind admiral.
arrow looping/raining is useless, nothing more then a flashy gimmick, and anyone constantly using said technique with him is only hurting themselves in the long run.
as for most difficult/easiest to use.
well.....the obvious answer for easiest, would be metaknight, hell thats the main reason so many folks flock to him.
for the hardest....kind of depends, even though they aren't competitivly viable, the answer for just literally hardest to use, would be obviously the lowest characters on the list, ganon, link, ike, ect, because they are always gonna be fighting an uphill battle.
in terms of being competitvly viable.....hm...i'd say probably wario and diddy, seriously....wario just has a really awkward style of play compared to most other characters in the game, so you've got to really know wtf you are doing with him if you want to win, but obviously its definatly worth it.
with diddy, you've gotta master his nanners, pure and simple, you HAVE to learn to seriously master his banana game, not to mention be very cautious cuz your own weapons can be used against you....