Honestly I would say this is the biggest problem with Mercy at the moment, current Valkryie Mercy is “balanced”, but balanced doesn’t mean engaging. Talk to any Mercy main from those that are left and haven’t abandoned her or the game and most will say she’s never been so boring.
She needs changes to her kit so she’s more exciting, Guardian Angel and resurrect most people are happy with, they really need to consider giving her a more thought provoking and engaging ultimate that isn’t a glorified spector cam.