Update (Post Media Tour):
Living Dead has been the subject of a lot of discussion since its release. With the Media Tour revealing buffs to two of the other tank invulns, I think that it's a good time to revisit this subject now that it is objectively the worst defensive capstone ability on any tank by a significant margin.
Here are the key problems with this ability:
1) Implementation.
Walking Dead requires you to receive an amount of healing equal to your total HP to remove the effect. There is presently no way to track this requirement in game. This is not the same as healing to max HP. So your healers are either forced to use Benediction on it, or throw heals at it until the debuff icon disappears. At bare minimum, this needs to be communicated in the UI, with a HP overlay to show how much healing is still required to cleanse Walking Dead. You could have a black bar that sits over both your personal and party HP bars that progressively shrinks as you receive heals. It's a bit odd that we have job-specific UI elements for relatively unimportant things, like the duration of Living Shadow, but for not basic features required for survival.
It's also worth noting that the mouse-over tooltips for both the Living Dead ability and the Living Dead debuff have been incorrect on English localization since 2015, which is why there are surprisingly many players that think that you need to actually reach 100% HP like the standard Doom debuffs.
2) Synergy.
Living Dead really is on the wrong tank.
On balance, DRK has the worst self-healing capacity of any tank in single target. That was true this expansion, and will be even more true next expansion, when PLD and WAR both have significant gains in self-sustain. Even DRK's strongest defensive ability, TBN, has no interaction with this ability, because barriers don't count towards meeting the healing target.
The loss of Convalescence in Shadowbringers, as well as the loss of Sole Survivor, both removed any semblance of control that you had as a player over getting healed through Walking Dead. And while I know that not everyone enjoys solo content like single player PoTD (or the Bozja duels), it's frustrating to play a tank and not be able to use your capstone defensive ability at all, knowing that you're completely unable to heal through it. This is part of the reason why I also think that suggestions to 'ease up the healing requirement' don't really hit the mark. You really don't have any agency whatsoever to help survive the Walking Dead effect. This is an ability that really should be designed around a burst self-healing tank.
3) Healer Interactions.
The single best ability for addressing Walking Dead is Benediction. This is really the only way that you can guarantee a full 9 seconds of Walking Dead. Why 9? You die when the cooldown reaches 0 seconds. Otherwise, you're just throwing heals at it until the debuff disappears, which could be anything from 1-9 seconds.
Living Dead is on a 5 minute recast.
Benediction is on a 3 minute recast.
The problem here should be obvious. For two abilities that are seemingly meant to be used together, they sync up very poorly. If you want to always use them together, you need to hold Benediction for an extra two minutes every time, resulting in lost Benediction uses over the course of the fight. This wouldn't be an issue if Benediction instead was on a 2.5 minute cooldown (150 seconds). There's also the question of how costly this effect will be for other healers. Benediction, at the end of the day, is a single cooldown. Other healers may end up using multiple cooldowns to eradicate the effect. And again, without a clear visual indicator for the remaining healing requirement, it's easy to burn more cooldowns than you need to keep the DRK alive.
4) Duration.
The active portion of Living Dead (i.e. the part when you actually mitigate damage) is always less than 10 seconds. Living Dead and Holmgang only ever mitigate damage when you're at 1 HP. Otherwise, all the damage goes through. The active portion of Living Dead is Walking Dead, and Walking Dead only.
I'll let that sink in for a second. Anyone who claims that Living Dead 'potentially gives you 20 seconds of invulnerability' has no idea what they are talking about.
Just to illustrate this point further, let's say that Living Dead was a trait that automatically activated Walking Dead when you receive an attack that would otherwise kill you, up to once every 5 minutes. Would you claim that such an ability makes you permanently immune to death? No. The active duration only starts when you hit 1 HP. (It's worth noting that this design approach would actually be a downgrade to what we have).
The real advantage of the 'Living Dead' buff is that it allows you to precast your invuln. So if you know that a tankbuster is coming at a particular timestamp, you activate it 9 seconds earlier. This starts the recast timer ticking, which shaves off 9 seconds off. You could say that, if you time Living Dead correctly, it has an effective recast of 291 seconds.
So what's the real duration of Living Dead? It's something between 1 and 9 seconds, depending on when your healers give you an amount of healing equal to your total. Heal too soon and the death immunity effect ends early. Heal too late and you die. It's the only invuln with this problem.
5) Balance.
So let's get to the key balance problem that Endwalker has introduced.
Regardless of whether you like Superbolide or not, there's a basic trade-off that you see between PLD and GNB. Superbolide has a shorter recast (by 1 minute), but comes at the cost of a penalty. With the buff to Superbolide duration in Endwalker, both abilities are otherwise equivalent.
Death immunity is a bit worse than true invulnerability in that you only mitigate damage when you're at 1 HP, so it's a bit difficult to compare these two directly with Holmgang and Living Dead.
Let's look at Holmgang and Living Dead, though, since you can draw a direct comparison between the two. Holmgang has a shorter recast (by 51 seconds if the DRK plays optimally - see previous section). But Living Dead is the ability with the penalty. And with the buff to Holmgang to 10 seconds this expansion, the active duration of Walking Dead will always be shorter than Holmgang. It's just objectively worse, even factoring in all of the previous design problems that we highlighted.
Wrap-up.
I'm glad that we're lucky enough to have a development team that takes pride in 'actually playing' their own critically acclaimed game™. But I find it extremely worrying that they seem to be clueless to many of the most obvious sources of gameplay complaints.
I don't really care at this point if they shaft DRK for a third consecutive expansion to inflate their new job numbers, but it would nice to see a few actual fixes on a problem that has been a thorn in players' sides for 6 years now. Acknowledging and admitting that there are major problems with this ability just the starting point.