PLD's stun is the shortest thus making it the most possible to stun pretty much every eruption in Ifrit. GCD doesn't affect that as I've done runs with every eruption stunned and I've done runs with them missing stuns.

So while people will happily say it's not the healer's fault, after a vulcan burst and the healer doesn't top everyone off and he Hellfires then follows it up with a seer > vulcan burst, you can blame the healer if people die. Not avoiding plumes, sure, that's tank/DPS fault, but dying due to never getting a heal or topped off? As much as one may not want to admit it, the AoE's Ifrit let out will eventually wear someone down.