I don't necessarily think that Mercy is straight up overpowered, but I still don't see how, when competently played, she won't an absolute must pick. I freely admit I could be wrong, because overwatch is one of those games where you don't really know how things will play out until you give it a month or two for players to adapt. But a revive is such an incredibly powerful ability that I don't see how it's negotiable to exclude it in any team comp. Rez is harder to pull off now, because she no longer has invulnerability and getting in to rez puts her in danger, so it's not like it's absolutely impossible to outplay.
However, that's because people need to relearn how to play her to an extent. Imagine the mercy in question is fairly nimble, can make smart decisions about who to rez, and uses her ult intelligently. How does any healer's abilities compare to that? Zen's discord is powerful, sure, Lucio's speedboost can do a lot, yeah, but Mercy not letting the enemy team die is hard to compare that too.
Like I said, maybe I'm wrong. But at the very least, this ability should mean that every team fight has players killing the enemy mercy first and foremost. I mean, it's pretty advisable to go after healers first already, but a Mercy on the enemy team makes her priority target number 1.