I can’t deny that, but if you already know one of her team mates is dead you dont necessarily have to baby sit the corpse, you can just keep an eye on mercy.
You see her fly towards the corpse, you start shooting and in many cases she’s half health before she even begins to cast rez.
And I get there are good mercy’s out there who are good at not telegraphing their movements, not all rez’s are preventable no matter how well you keep track of her (throwing you a bone as you said you don’t think it’s OP, it’s just annoying, fair enough).