Healing is a lot harder overall, because the other classes are very limited in their ability to correct mistakes. If someone screws up and gets hit by a fireball in O1s, only the healer can save them and its a split second decision. Maybe its a healer/dps + tank argument-the latter really have it easier due to it being self-contained more.
It's harder than you think. The reaction time is pretty small because it can cascade to kill the next person even if you heal it, since it leaves them at low hp enough.If someone goes wrong way with thunder mark, it's easily doable to recover.