Try fighting against LA Ryleh where his dominion or spells cause commanders to go insane. I have had my own insane priests preaching against my dominion. Worst part is that I cannot order them away, or disband or kill them.
The overall point is that a prophet/heretic is best utilized on the fringes of your territory, mostly where the enemy has dominion.
If kept in your own lands the prophet's dominion spread will be hindered to a degree. The good news is that the two effects resolve separately. So a heretic and a normal prophet in the same zone won't effect your dominion any differently then prophet/heretic of the same heretic level.