I have a priest and havent stepped in the arena shadow since s4 because of the burst dmg. What needs to happen or i hope that blizz would do is buff dispersion a little more.
Give it something a long the lines of a fear effect if someone is attacking you while in your shdw ball. Or put a blind effect and a dmg reflection ability to stop rogues and warrs from just beating on you throughout and your dead right when you come back out of it.
And to make it even easier you can make it a glyph so you dont fear mobs around in pve.