Obviously, only Sebazy can speak for themselves, but I believe what they meant with "healers should be immune from failure" is that their gameplay is more or less intentionally designed to be difficult to fail at.
Healing requirements can't be too high, because that would make healers maybe struggle, and that would make them sad.
DPS can't become necessary in everyday gameplay, because then healers might mess up balancing damage and healing, and that would make them sad.
Managing resources in the form of gauges or self-buffs can't be too complex either, because then healers might get overwhelmed, and that would make them sad.
And so on. It all feels very childproof and that Squeenix, plus the community, seem to think the average healer are morons that can't handle any form of pressure, complexity or depth. Which is well, actually sad. Sorta limits design space.
