Overwatch's angelic support character, Mercy, can't seem to catch a break. In the latest patch Mercy has taking a 17% reduction with her healing beam. You can find the full list of character updates here. This will go into affect when the latest patch hits. Developer Geoff Goodman posted the update and gave Blizzard's reasoning for each tweak. As far as Mercy, here's what he said:
Mercy is intended to be able to consistently pump out more healing than any other healer over the course of a match. While this is currently true, the difference in healing is so significant that it makes it very difficult for other healers to compete with her for a spot on a team. Reducing her healing output will close this gap a little, but she will still maintain her status as the go-to pick for raw healing power. We’ll keep an eye on her to make sure she is still a strong pick.