Mei is such a frustrating character in general.
If you think about her main weapon, it's kind of antithetical to the concept of combat as a whole in the game. Overwatch isn't like Call of Duty where it takes just a couple of bullets to down an enemy. Some characters obviously make mincemeat out of others, but on the whole spotting enemies, trading some blows and continuing to engage, retreat, maybe use the geometry around you to your advantage, are generally your options.
Mei doesn't have any of that. If she engages you, by definition she removes both your mobility and your ability to attack. Your options are reduced to zero. You can't reciprocate in any way either, since you shooting at her while you start to slow down doesn't do anything to stop her from freezing you. In other words, if she engages you, literally the only way to prevent your death is to outright kill her. The fact that she can very, very easily force you into this "you or I die, NOW!" state makes her an incredibly odd choice for a character in a game like this.
I'm not gonna be calling her OP or whatever, because there's no real reason to believe she wrecks everyone else in the grand scheme of things. But conceptually she seems confusing. I'm hard pressed to find Blizzard's intended rationale in terms of what an encounter with an enemy Mei is supposed to be. She has a one-dimensional "I will freeze you" relationship with every other character in the game. They all freeze in the same way/rate and you kill them in the same way once they're frozen. Molten Core'd Torb is just about the only character with any semblance of a counter, and it's not even that I'm looking for a counter so much as any sort of "wrinkle" to give the freeze ray any kind of nuance or dimensionality with regards to the other characters.