Hey

Mei has plenty of pros and cons.

She's strong with the right set up (Water/Fire).
However, she's pretty dependent on combos and her mystery costing 60 is a hinderance with certain lineups requiring her to have the right set up.

Against Mei teams, it's pretty predictable to who's the threat and that's the massive fault when it comes to Mei. You have to protect her.

I'll suggest asking the community on teams that can counter Mei centered teams.

Hopefully, this helps a bit. PM me if you have further questions