Gonna agree there with Megadodo.
Mercy doesn’t have that much concerns with breakpoints.
Her issue is pocketing Widow and Hanzo (and formerly Sojourn).
Because those heroes violate formal counterplay guidelines.
Counterplay
(According to the most senior gameplay designers at Riot and Valve)
“After you make your Choice, your Opponent should have the capability, to respond with their Choice, which includes the awareness that a Choice exists, and enough time to understand and react with their Choice”.
https://www.teamfortress.com/post.php?id=2477
https://leagueoflegends.fandom.com/wiki/User_blog:JAlbor/GDC_2013:_Teamplay_and_Counterplay_in_League_of_Legends
So the way you keep them in check, is by making them dead more often. (Or the equivalent downtime)
But if they don’t die, because of pocketing, that’s a problem.
Since that “Glass Cannon”, just becomes a “Cannon”.
Easy fix for that:
WIDOWMAKER
Passive: Widow’s Curse
- All healing received is reduced by 50%.