Post Preamble:
Give it a read, it won’t hurt you _
Its not that Mercy is underpowered. Others say it this way because that it what may feel like.
However, that feeling is rooted in the issues that Valkyrie takes away from player engagement in the game.
Resurrect makes you feel helpless while using it to the point that it feels better to just do “Hide n’ Rez” or not do it at all because the teammate you Rez will not likely make good use of it or at least you don’t know that they will.
The feeling Resurrect compounds and issue of Mercy’s dependency on teammates to make her use meaningful.
That wasn’t technically different in the past, but with Mass Rez a Mercy Main had an ability that you could see the immediate result of your choice, and if it was countered the immediate failure of the choice.
That spontaneous was enough for many to love playing Mercy because the rest of time it felt like you were the underdog until that one moment.
That one moment that is at this time no more.