Oh gee. If you’re going to claims something like this, I expect some evidence to be accompanied with it. As of now, 1-man Res is less balanced than mass Ress. How do we know? 2 things. It’s an ultimate being forced onto a cooldown ability and again, it is on a cooldown ability. Mass Ress, on the other hand, is more balanced than 1-man cooldown Res simply because it is an ultimate. It is earned.
Edit:
Also, she doesn’t look balanced according to statistics. Statistically, if you are playing Mercy right now then you are deranking, and if you’re playing her in high ranks then you’re throwing.