My thoughts would be they're having trouble on the first phase where the napalm AND the massive Tank damage from spellfire will put a strain.
The napalm is healable yet it also depends on your raid members perogative to stay spread apart as having more than 1 person sprayed will put massive strain on raid healing. No amounts of castable hots would save someone if more than 2 at the very most gets napalmed.
The spellfire damage is dependant on external damage cooldowns. And reactive healing to be done soon after. The damage stacks up over that short period because it's half-second ticks of 10000+ damage.
Paladins have problems because of two factors. The healing is spread between a tank beacon and napalm healing. The beacon on the tank makes one part of the problem moot but then again there's a lag factor to consider between the direct heal we do and the beacon heal.
HoTs rolling would be one thing. Then it becomes a mater of pre-mediation of damage which isn't too good if you seem to miss the guesswork a lot. Best to be safe with an actual heal than guaranteed heals rolling from HoTs and then popping an extra-boosted heal spell on top.
Healing is the source of life. Without it, you're left with death.