As some already said. The argument that it was unfun to play against is void because of the way how unfun she is to play now and how that fact is being ignored.
So many other things in the game are very unfun to paly against. In that, you can count all heroes which can stun, OHKO (Stuff which can kill you extremely fast is on the same level), knock you around and have insane movement. That involve almost all heroes in the game.
We are playing a PvP game at the end of the day, so every time an ability is killing or outplaying you, you will have less fun than the guy which won against you.
So… with that being said, in a game like OW, fair to play against goes over unfun to play against by a long shot.
Just a few examples for that are Genji, Reinhardt and Ana.
Genji is extremely unfun and frustrating to play against, but if you play smart you can hard counter him or out play him. That’s fair gameplay.
Reinhardt is a monster in CQ fights, especially if you can’t get away from him and his charge can feel very bad as well if it hits. But again, stay far away and he can’t really do anything against you. Have tons of burst dmg and CC and he is nothing. Even his charge, considerably the most fair OHKO in the game, can be avoided quite easily.
And Ana, a very strong hero which can kill almost all ult’s with Sleep dart and has by far the best healing in the game + Anti heal. Her power is still fair because it’s so hard to get value out of it.
All 3 can be frustrating and unfun to play against in mayn ways, but because they are very fair, it’s not that big of an issue.
Why is that important? Because the same was the case with Resurrect Mercy.
Healing not the best but continuative, making it good for sustain dmg.
DmgB a strong thing which needs a good amount of game understanding to use properly.
GA/AD back then very hard to utilize but also very useful.
And of course Resurrect.
Very strong but also very easy to outplay if the enemy is smart and knows what Ult economy is and quite easy to counter with AOE Ult’s.
A major part of Mass Res was very fair to play against, on the same level as the previous heroes.
She was never to strong or to weak, but just balanced. Even with invulnerability.
The only real issue it had was the fact that it had no preemptive counter play and that was something which is very easy to fix, to make once again fair.
“Unfun to paly against” is not a viable reason if the fairness is not considered.