I mean I played Old Mercy, and also dabbled in the other supports, but for argument’s sake I am going to use the two heroes I have with literally less than an hour of game time separating them: Mercy and Zen because they both have sustain heals, but the impact the individual brings is completely different even without touching the fact Zen deals damage.
Both their base healing is nothing to write home about 60/50 hps and 30 hps. It gets the job done but with other supports in their category they might have the highest (or second highest) healing per game in their respective category, but the impact of those heals aren’t anything to write home about.
Getting slapped in the face with a Zen orb can be a “oh thank you!” moment but you have to chill for a little bit and let the healing do it’s thing, same with Mercy beam. Ana, and Lucio get a boost to their healing which makes it feel more in the mid-fight. Ana has a base 75 healing per bullet, 100 burst heal with her nade that then grants a 1.5x to any healing to that target. Lucio’s amp though is close to Zen’s single target heal, could at least heal multiple people (and has great synergy with Ana nade). Also, Ana gets about the same amount of healing as Zen, the reason she is a main healer is because of how the burst of her heals work.
What Zen has that Mercy doesn’t is an ultimate that kind of makes up for this subpar consistent healing. His ult heals for 300hps within a 10m radius. Mercy only gets +10hps within the same radius. No wonder a lot of Mercy’s feel that this is “meh” in terms of power. Same with damage boost Orisa can do it better (50% vs 30%) and she can still contribute to the fight with her pew pews.
Mass Rez was Mercy’s Trans. Not only that, but mass rez was the counter to ult stacking and it of itself was countered by things like D.va Bomb, and Tire, and now Hammond. The counter to rez was to know your ult economy.
It wasn’t without faults though, and personally at this point I don’t care if it comes back. Seeing how the off-supports have more defensive ults and main supports have more offensive ults, I’d actually prefer that it didn’t. I just want more zest in her kit. Personally I would like a rez-version-of-nanoboost. You rez a single target and they are like nano’ed. Basically make the Ana/Mercy ult relationship like Zen/Lucio and make it instead a choice between which type of utility (meaning we’d have to give Mercy a new E preferably something that makes her better in the mid-fight) and types of healing (burst vs sustain) would work best.