What Domqe said. I don’t know how you play Mercy, if you play her, and if, how much you played her since her nerf (which was partly reverted on her ult, Domqe explained it), I have to say that you practically don’t have time to do something other than healing. She can’t keep up with burst damage incoming on single targets. No surprise there when you have Winston outdamaging her healing (and he is never about burst). When I play her, I barely can keep up one target alive, any more simultaneous damage incoming on the team (ults, high damage abilities) and your inability to function as a main healer becomes obvious.
There is simply no time to do anything else, I manage to boost my teammates sometimes, but it became very situational at best.
Mercy is not supposed to be a burst healer, or an aoe healer, but her single target heal was overnerfed. It needs a slight buff for her to be viable in her main role.