Metaknight definitely is hard to use, or rather, he can't really kill. Matches last longer in Brawl but Metaknight in general just can't kill. His only decent killing moves are his Up-B, Nair, and his smashes and even then, your opponent has to be at least 130%.
The hell are you talking about?
His dsmash can kill at 100% if the stage is medium sized (battlefield)
His nair, fair and especially dair can be used off the ledge for great edgeguard
His upB can kill anyone trying to edgeguard you and is a staple finisher out of upair combos
His Fsmash is laggy but can kill at high percentage
And what makes up for his "slightly" lower killing moves is that you can SPAM his kill moves without really worrying about being punished. Like I swear, I just dsmash all day when they're at killing range since Meta's moves have almost no lag. And since wavedashing is out, its even more hard to punish meta.
Just by the fact you didn't mention his dair makes me think you just watched videos and haven't actually played.
EDIT:
I have one general question to the people in the Meta boards. When you say that Meta has no killing power, do you guys take into account everything else? Like how he has no lag in his aerials, how he has great Bmoves, how his aerials have great range and speed, how his moves hits behind him, how he has multiple jumps, how he has one of the best recovery in the game?
So with all that pretty much set in stone (anyone who says meta has slow aerials and slow ground moves have no clue what he/she is talking about), if Meta has any one good kill move (he has dsmash, upB, fsmash, nair) can't he make up the slightly less knockback by how he can rack up damage incredibly fast that THE MOVES THAT DOESN'T HAVE TOO MUCH KNOCKBACK NOW HAS?
Play the game before you speculate. Videos of guys playing CPU's aren't indicators to anything.