Meta Knight isn't THAT overpowered exactly, just that he's seen to have zero disadvantageous matchups in pro play. So if you're picking him for that level, there are no "what if my opponent goes with"s. The worst that can happen is Meta Knight vs. Meta Knight. It's like bringing a gun to a knife fight. Or at least a sword with pointy branches. Not nearly as extreme as with SSF2T, like Warp said: Akuma is more like bringing a nuclear warhead to a game of duck-duck-goose.
Don't worry about it if you're not a pro player though. You can joke about it with your friends, yeah, but at the end of the day, what happens on those higher levels don't mean squat when you're on Pirate Ship with items flyin' around and four players staying up way later than they should. Especially considering that the game itself was more or less balanced taking Final Smashes into account. Pro play is really very very very different from what you and I are used to, so if you're thinking "I never thought character M was that great or that character G was that bad," that's why. What makes a character top-tier in advanced play is simply not seen in normal play.