The biggest issue with metacritic is that many people don't understand what it actually is, and end up using it as a universal quality indicator.
Some people also take issue with that they use a weighted average rather than a pure one, but I don't personally think it really matters. It rarely results in a gap of more than 1 point (which is less than general variance will usually cause), and to be honest I don't understand why MC don't just ditch it, given how little it ends up doing.