McCree, Junkrat, Tracer, and Genji basically have a guaranteed kill on her as soon as she pops out of her Cryo-Freeze, and she can't stop the likes of characters that can dash out of her Endothermic Blaster (Tracer & Genji especially). Yes, her ice is frustrating, but it doesn't do a lot of damage & can be countered by a good healer.
Really, with the exception of her limited crowd control ability, she has nothing that the other Heroes don't do better. Mei can be a liability than an asset to the team, as she's taking up a lot for a good player with actual killing potential. I'm not implying that "Gosh, she doesn't carry the team" , but Mei doesn't bring a lot to the team in comparison to anyone else, even in a controlled environment. She doesn't pose a major threat to any of the heroes of Overwatch.