It could also just have to do with the amount of damage that the AoE soaks on the split sides do. Even on heroic, that damage is really sketchy if anyone has bombs at the same time. On mythic, I'd imagine it's insane, and the extra damage reduction from Guardian affinity + Ekowraith might have been needed. I also wouldn't be surprised if that's why they explicitly used an alt to stack 3xRDruid for that fight. Soaking in bear + Ekowraith gives a ridiculous amount of survivability relative to other specs, and specs with weak survivability like HPriests might have had issues living through those soaks at week 1 gear level and their strat.