First, if you don't know about tanks... don't comment. For real dawg this is how we get some stupid "fixes" that people want.
Second: DRK's self sustain is peanuts compared to WAR. Abyssal is ABYSMAL for raids. You never use Living Dead for dungeons, my dudes... unless you have WHM with Bene which indicates a huge problem out of the gate already with having to throw the biggest heal at the invuln for it to be "okay"....
DRK has Soul Eater, 3gcds for like 12k hp heal at a critical at Edensverse best in slot... not great. You get ONE soul eater for the invuln time you have for Walking Dead, when tanks have 160k hp. Abyssal is like...10k as well. So impossible to heal it.
Warrior on the other hand, can easily HEAL THEMSELVES TO NEAR FULL after taking the hit with holmgang, making them never in danger of dying from their invuln. Their invuln sounds like total aids on paper, but in practice, it's the strongest one because of their sustain kit of Equilibrium, which if it crits at edensverse bis is like 70k, Thrill, which increases their hp and heals it, gives them 10 percent heal up buff for 10s which ALSO EFFECTS EQUILIBRIUM BTW!! And let's not even get started with how nutty Nascent is... you can heal over 80k hp one ONE use of it... .two infuriate stacks at 35k to 40k each and upheaval, on a 30s cd, that depending on where it's used either heals 15k or 7k ish whether in IR, or you just crit it. That's at least 84k healing on top of potentially 35 to 70k from equilibrium, and can be boosted with thrill of battle. You are NEVER DYING with holmgang.
Superbolide can be paired with Aurora, 1200 potency of healing and you are INVULN so you don't take chip damage, it just heals you up during that time, for a full clemency's worth of hp, as they are both 1200 potency as well as equilibrium.
You can also proc excog off of superbolide as you meet the requirements for it, and an astro can chuck a 1100 potency Essential Dignity at you, as you are at 1hp and they gain max potency for it. Healers have to use almost NO resources for other tank invulns when done properly. Not for WAR as it heals itself, not for GNB, as it has aurora, probably tank swapped so it's not getting auto'd and is healing with aurora, and not with PLD as it doesn't take any damage.
DARK KNIGHT IS THE ONLY ONE WHERE YOU NEVER WANT TO USE THIS SHIT. IT IS GARBAGE. As Xenosys Vex put it: "What a selfish skill... it's a black hole for healer resources..."