I never experienced mass-rez in overwatch before, but I have played a cheap copy of Overwatch and that game has mass-rez.
To me, mass-rez isn’t as powerful as you think it is. I mean, sure you can counter most ults in the game with a single push of a button. But mass-rez really depends on whether you got a good teammate or not in my opinion. Not to mention where they die as usually it is caused by bad positioning and mass-rez is basically a feeding ult in that type of situation.
To me it’s not that powerful and has a lot of counter plays. I mean they literally stood still when they are rezzed and the enemy team has all the good positions while my team has the bad ones.
Not to mention it’s only good if your teammates is actually can stand a chance against the enemy.
I prefer Valkyrie over mass-rez due to said reason. At least Valkyrie can be used for peeling when your team is in a bad position and it’s also very flexible while mass-rez has a very specific requirement and it only goes as really good or really bad ult, there’s no middle ground for it.