Let’s look at Mass Resurrect first.
Mass Resurrect’s fatal flaw relates back to it being frustrating to play against. Now I know what you are thinking “What ultimate isn’t frustrating?” I agree… but it goes deeper.
The difference is a fun little part of human psychology… Loss Aversion. Humans absolutely HATE it when they feel like someone has stolen something from them. Whoever got those kills that got resurrected usually felt like they earned them, they were theirs… aaand Mercy just flew in and stole them all. Games are designed to engage and entertain humans. Having something in the game that goes against a basic part of human nature is… not a terrible idea per se, but it should absolutely NOT be done lightly and if done, should be the game’s main thing. Playing with the player’s loss aversion is not the point of Overwatch, so Mass Resurrect had to go.
Valkyrie has a different fatal flaw. It’s flaw stems from language.
Language is incredibly important. Like… ridiculously so. I seem to recall an instance where Blizzard implemented an exp penalty for being logged on too long and players complained. They renamed it to bonus rest exp and… everyone liked it. Same mechanic, framed differently, completely different response.
Valkyrie is Mercy’s ultimate. What does the word ultimate mean? Well if we pop over to www.merriam-webster.com, definition 1-c seems to be the most applicable to our situation. It reads “the best or most extreme of its kind”. Does that description fit? Well let’s look at the things it does to try to answer that.
Valkyrie does AoE heals. Is it the best at that? Trying to claim that Valkyrie is better at AoE healing than Transcendence is a very very hard sell.
How about Team Damage boost? Orisa’s super charger does 20% more.
Flight? It is debatable whether Valkyrie’s flight and Pharah’s flight is better.
Diving people? Dragon blade and Pulse bomb are better at that. Even DVa flying in with all guns blazing is probably better.
Getting back to point to contest? The entirety of Hammond’s base kit works way better for that.
Valkyrie is not “the best or most extreme of its kind”. It just isn’t. Players expect it to be though… because the game is calling it an ultimate. It also cannot be buffed to be ultimate in one of its current use cases because that would invalidate a different ultimate in the game.
tl;dr: Both of Mercy’s ults are bad and she needs a rework.