I've been thinking about wifi and comparing all the details and what not. And really, on wifi, how a character can perform on wifi is determined by the player him/herself. Finding ways to abuse wifi isn't that difficult. Rolling, smash spamming, and camping are simple ways to abuse wifi. However, if the player doesn't abuse wifi, then the character isn't so broken.
Let's use Meta Knight as an example. If you spam nado and dsmash (and I mean even more than offline MKs do lol), then winning isn't all that difficult if lag is bad enough to ensure your opponent can't punish it. However, a Meta Knight that tries to move a lot and mix up his moves is a teensy bit harder to use due to him being harder to control in lag.
So really, it depends on just how much the player is willing to abuse wifi and not as much the character they use. Like even a character that's bad in lag, like Marth or Sheik, can win if the roll all day and spam smashes >__>
I lol at wifi sometimes xD