I don't think a good game engine and mechanics necessarily means unbalanced games... if Melee had 26 viable characters it would still be a good game.
And until Smash 4 gets polarized, it's mostly open up top (no one has identified a "THIS CHARACTER IS CLEARLY AMAZING" like with Sheik (Melee) and MK and Snake in Brawl) so it's possible there will be better balance, but I think Smash 4 is already able to be stated to have good mechanics and design [my opinion].
But I mean, if we just broke ROB, Lucario, Dedede, Falco (either buff air speed a bit so that fair chains reliably or remove fair, replace with nair, and Melee nair, + better ftilt, reflector cancellable whenever, to make him truly stupid), and perhaps Ganondorf and Link, would you still be happy that you could play those 6 but not Charizard or Fox or Squirtle because you just get bodied by the top 6?
I know I would only because I have 4 of the 6 I like covered ( I enjoy Falco, and sort of enjoy Lucario and Link and Ganondorf, but if you swapped them for Wario, Yoshi, Zelda, and Roy, I wouldn't), and I think most people who don't main one of those 6 wouldn't like it either, being forced to drop who they like (ex: Charizard) for someone viable if they don't want to get stomped by players of the same skill level. That's why I tend to favor balance (at least in games that are mods/updated) over trying to make a few really crazy fun characters [since again, I think it's easier to make characters more fun than it is to try to make them balanced without becoming not-fun].