Why the hell it woudn't? Deadeye deals ~250 dmg for every second you charge it, so if McCree waits for ~2.4 seconds he can deal 600 dmg.
The duration is 6 seconds so he can deal up to ~1500 dmg.
As Roadhog, you can predict the time he's gonna shoot so you can heal & get damage resist at the same time he shoots to survive the hit.
(Because you know, McCree is P R E D I C T A B L E)