Worrries about THAII'm not the one to fight PBG (I've never sold any), but with that stated, the doubters would stress:
1. Controlling the direction of the fireflood. If the fire burns in the wrong direction, you don't get much oil. PBG answers that the only sink is the producing well so it must move in that direction.
2. Premature breakthrough. If the fire proceeds too fast and reaches the heel too soon, there will be limited production. This and item 1 have caused previous firefloods to fail. PBG responds that those lacked a horizontal production well.
3. Burn-out, perhaps caused by a lean zone. If the fire goes out, you need to preheat and ignite again. You might need to drill a new horizontal well closer to the fire-front before burn-out.
Anyone else?