Yeah, she's annoying, and I keep getting the impression that when I play her I can't slow people down for shit, but when she's played against me, I freeze in like 2 seconds. It's a conspiracy!
I think the annoying part really comes from the freeze effect being a bit fuzzy in terms of feedback and progress etc. and the fact that once you're frozen either by her or her ultimate, oftentimes you're basically already dead but waiting for it to happen, and that's not a very enjoyable situation. At least McCree's stun is shorter and you don't get this 'oh crap, here it goes again, now she's going to get close and aim at my head dead on and I'm going to die' stretch of a few seconds