Mercy had a shitty kit from the start. Playing her teaches you the WORST player habits in a game like Overwatch. That's why we get hella players who don't know how to get on point because they learned to play a passive character. Mercy fans just don't like the fact that they're gonna have to work to rank up now. Idk how deep it goes internally but the game still rewards mercy for doing the most mundane shit. I still see Mercy potgs where they sit behind a wall and rack up assists when in reality our secondary heals (if we even bother to get one because for some reason people still see Mercy as a solo heal god) manages to give out most of the sustaining health.
Also REZ IS A FUCKING JOKE.
Why pick a character that has to switch between ONLY healing or ONLY firing a weapon (no matter how fast you can do it. If you're not healing, you're not keeping your team alive and if you're not doing damage you're not helping your team push.
Every character has balance. The reason Mercy has the ability to bring other teammates back from the dead is because she herself can't do much else aside from pocket heals and whip out her pea shooter every now and again to finish off some weak enemies. I hope Blizz does make some changes to her enough to where people stop playing her as often because she really isn't that good of a character in a game like this and there are better healing characters that WILL KEEP YOUR TEAM ALIVE SO YOU WONT HAVE TO RELY ON REZ.
Maybe I'm just a biased flex/support main but yeah. I'm just sick of Mercy still being a must pick after so long. No disrespect to Mercy mains, I'm sure a large deal of them are good healers and don't behave like the ones I mentioned but there's just far too much Mercy play in the game and I feel like it's one of the biggest reasons we don't see a whole lot of supports (they don't wanna play must-pick Mercy) or tanks (they don't trust their heals to keep them alive in heavy teamfights).