There are many problems with Living Dead.

The ability description and tool tips are incorrect. It's also one of the longest descriptions for an ability in the game. Imagine a new healer trying to figure out how it works mid-combat. Fun.
There are a lot of misconceptions about the healing requirements and duration.
There's a lack of UI information to support the use of this action. Healers have no feedback on how much actual healing is still required (you don't need to be at max health to clear the effect).
It's the only invuln that can be ended early by another player. An early Benediction can be just as dangerous as a late one.
The healing requirements are getting progressively worse, especially with the removal of Convalescence.
It doesn't sync up well with Benediction, which is required if you want to get the maximum duration out of it. If you reserve Benediction for Living Dead, you miss chances to use Benediction.
It's objectively worse than Holmgang. Put the drawback on the shorter recast invuln, and on a job that actually has abilities to self-heal through it.

All in all, it's a terribly designed ability which is convoluted for the sake of being convoluted. It needs to be rebuilt from the ground up. Failing that, just scrap invulns altogether and call it even.