And that “Undo” button could have been made balanced. They told us that they had a dozen different variations of mass rez that they planned to try out and we never got to see any of them. Rez is such an important part of mercys kit, at least to the devs, that they have fully stripped and gutted her game play to keep it.
The “Undo” button was only a problem After they took away all of the counter play other than finding and killing mercy before the team fight even took place. It had no counter play to stop mercy in her tracks of using the ulti like so many other ulti’s do have. That could have been fixed. But instead we got an overpowered monster that dominated for Months and months.