The main issue I believe people have with Mercy is that she got a rework that their mains didn’t ask for and it was more to appease the people complaining about Mass Rez (which historically made her underpowered - C or D tier), and then afterward she got hit with nerf after nerf so her mains had to work with some new limitation every two weeks or so.
Then, they had decided to try to compromise and suggest that they either nerf her output or buff other healers’ outputs. Instead, they did both when in reality, only one of those was probably needed.
Ironically enough, there are plenty of abilities and ults on live that can counter Rez.
RIP-Tire, Self Destruct, Molten Core, Minefield, Hack, EMP, Shield Bash, you get the idea.
She went from a healer that was underpowered but unfun to play against to a healer that is balanced but unfun to play as for a lot of people. She’s essentially back to her beta build, just with a few bells and whistles attached. Her healing was buffed to 60HP/s before because she couldn’t keep up with Ana. So I don’t think her healing was an issue.
Then there’s also the fact that those Mercy mains want her to have some impact on her own. That’s why they tend to not accept statements like “X hero is a buff to Mercy” because they want her to be more than just a pocket. If anything, she shouldn’t have to just pocket the carry DPS, but “pocket” the entire team, so to speak.
It may be “small” compared to how many people play this game, but it’s still a large enough sample size to see some trends - that pockets in each group, not just Mercy mains, but other support mains, tank mains, and DPS mains all believe that Mercy is unfun to play as and needs something. Still doesn’t change Blizz or pros’ thoughts, sure, but it showed that it’s not just Mercy mains that think she’s unfun.
There was some fun in the swing potential, sure, but I think the -10hps nerf was the last straw for many because her healing output was never an issue in the past. So why nerf something that wasn’t a problem? And now, with her 50hps, she’s in the same boat as the other sustain heroes like Bastion and Soldier. Ever since dive fell, it’s just been burst, burst, and more burst. The game doesn’t have a place for her and the other sustain heroes anymore because the past few metas have been nothing but burst.
Her interactions with Ana changed since her rework too. Before, in S2 and S3, you could play Mercy and be fine. People preferred Ana, but if you could only play Mercy, the team worked around it. You were able to climb with Mercy just as well as you could Ana despite Ana still being meta.
But now? On ladder, the mindset is very much a case of “You better play the meta supports or you’re throwing.” Back in dive, nobody wanted an Ana and if you picked Ana you were throwing because we need that Mercy/Zen combo. The same thing is happening now. People generally don’t want a Mercy on their team. They’d want Ana/Zen or some similar combo because that’s what is meta. When before, people didn’t prefer Mercy, but they still worked with her. But that mindset is probably an issue all on its own.