I've seen too many plays where a heimer sets up 5 turrets where the collapse ends then the turrets are out of reach taking no damage in the out zone and just death squad people on the center. The collapse should hurt everything inside of it including the hp on heimers turrets.
We're currently prepping a change for 13.15 so pets + summons will take damage in the fire. It will additionally add ramping grievous wounds to keep people from perma healing inside it.