Mei has her ice walls that can change the outcome of a game, a self heal that makes her invincible, and unlike McCree, she doesn't have a cool down on her stun/freeze. She's fine. More than fine honestly. McCree is used because he can take down tanks which is invaluable in competitive. Once he loses that, and he will with the nerf coming, they'll be on equal terms.
Hanzo though. I'm not sure, he doesn't really have an option if enemies get close besides a hopeful scatter shot. He does have a quick Ult, able to jump while charging, and can climb walls over Widow but I do think he could use something more. If his damage was just upped on his full charge and a lot more on his uncharged bow, he could be pretty viable. Something like 150 damage on body shots at full charge and 50-75 on no charge?