Mercy doesn’t require mechanical skill to play. Ana requires very high mechanical skill to be effective.
You cannot have a hero that requires no mechanical skill whatsoever have the same (or actually higher) output as a hero that requires quite a lot of mechanical skill. That is poor balancing, the latest changes address that and in the right direction.
What would be the point to pick up a hard mechanically skillful hero when you have a character that does the same or better with no mechanical skill whatsoever?