From a GM-level Mercy player, if you feel like Mercy isn’t viable in most situations or is “weaker than all the other supports”, you’re probably just not very good with her. She’s not as easy as you think, she’s not a niche pick, she’s not an “off-healer” (also labeling supports is pointless), she is more capable of solo-healing than any other support, and, if you’re playing Mercy outside of a tank-heavy comp, you should absolutely be out-healing every other support.
Right now every support is viable and excels at something. None of them are useful for everything, but all of them are great at something. Mercy is arguably the most generally useful support; She just doesn’t work well with the current meta or any tank-heavy team comp. She works best with spread-out comps and DPS-heavy comps, but she’s highly flexible and can be used effectively in most situations.
People often focus on meaningless stats like “other supports can do more than 50 HPS” but that doesn’t really matter if they don’t actually do that. Yeah, Ana CAN out-heal Mercy, but she’s probably not going to. She’s not going to make all her shots, she’s not always going to be able to use her grenade to heal her team, and she’s very limited by LoS, her mobility, and having to reload. The fact that she CAN do more healing is meaningless. Same goes for the others, but for different reasons. Mercy is almost entirely uninhibited and can almost always pump out a steady 50 HPS to whoever needs it most wherever they are. Her consistency is part of her strength.
As I spend most of my forums time debating Mercy, I’ll be happy to discuss specifics further with anyone who wants to.
TL:DR: Mercy is great in the right hands, mediocre in average hands, and ineffective in the wrong hands. She doesn’t need big changes; You just need to git gud.
P.S.: No, my profile isn’t hidden. Click the other tab.