Yup on leaderboards.
I feel its a weird skill floor for each. Flanking Mei's are super annoying just as McCree, but getting rid of a Mei is harder and wall helps a lot for stalling / control / negating ultimates / getting team pickoffs. Same for freezing a tank, lets the team get headshots and mitigates damage (on no CD!).
High level snap shotting roaming McCrees are scary, but a great Mei, if it fits, its quite good. It's just McCree can almost always fit in more maps more of the time.
As far as good Mei's I've run into it's just 2 in 18 levels, but I'm playing in stacks so the coordination is higher which might factor in?