Except, you really didn’t. You wanted ultimates that got immediate value with a simple Q press, the same immediate value that Rez got with her Q press.
EMP, Blizzard, Molten Core, Transcendence, Rally, Minefield and a couple others all require a similar thought process; be in or near the middle of the enemy or allied team and press Q for an instant advantage.
EMP: Go in, press Q, instant value
Blizzard: Throw it anywhere and the enemy can no longer have that spot of the map, instant value
Molten Core (new): Throw it into the enemy or onto the point and they can no longer survive on that spot; instant value
Transcendance: Use in the middle of team and now it’s much harder to kill them; instant value
Rally: Use in the middle of the team and now it’s much harder to kill them; instant value
Minefield: Use in the middle of the enemy or on the point and they lose that space: Instant Value
Mass Rez: Press Q in the middle of your dead team and potentially the live enemy team: instant value
All you’ve said is some mumbling about how the team may have to follow up, but once Mercy revives her team they still have to follow up to win.
EMP countered by Seperating doesn’t work if they don’t expect the Sombra to come in. We know by know more or less when Zarya has Graviton, she plays differently. So does Tracer, for that matter. Sombra plays relatively the same.
Trans countered by Anti-nade requires a specific hero out of 28 or 29. What do you do without Anti-Nade? Burst damage, but what if you don’t have a Junkrat or a Hanzo or a Pharah or something? What if you only have Soldier and Genji?
Mass Rez was countered by ult economy; even if she was standing behind her team and reviving two people that died out of position, which is an appropriate use of Rez, High Noon, Death Blossom, Rip-Tire, D.Va Bomb, Tac Visor, Dragonstrike, Graviton, Molten Core, Blizard, Configuration: Tank, Earthshatter, and a few other ults that I forgot the names to right now can all kill those people before they get back to safety. You either use the “skill” that net you the teamwipe to begin with to kill Mercy before she ults, or you use the ultimates left over (since your “skill” killed the enemy, no ults should have been used) to kill the rez’d heroes. It’s not countered like most other ultimates, but it was still countered.