Let's actually compare match-up charts shall we.
BlazBlue
Brawl
Rachel has one match-up where she is slightly disadvantaged. The rest are all good and most of them are 60-40 with two 70-30. She has a 45-55 against one other character who is also top. She pretty much dominates the rest of the case. Meta-Knight is slightly advantaged against a good number of the cast, but has a lot of evens as well. his overall ranking is that he has an advantage where most of the top characters have a slight advantage. Note that against none of the top characters does he have a large advantage and many of them are slight if not even. It only gets bad against the bottom of the cast, but they get stomped by the top anyway. Basically, it is not unwinnable and Meta-Knight can be beaten as very few of the top characters really have a large disadvantage.
See, there is a difference between being over powered and broken. Broken means there is no other reason to pick another character over them. Over powered means they are really damn good. Meta-Knight does have a lot of advantages, but very few where he dominates. He is no different than Sagat who did very well against the top cast. meta-Knight instead does Ok against the top, if not has a slight edge. Of course, better play mitigates it. Improving of the character's meta-game will also mitigate it.
Basically, I'm saying that the arguement are kid of bull. Yeah he's strong. Is he as unbeatable as you all think he is? No. This is more likely a few players being upset that they can't deal with
Mutas Meta-Knight.