DRK feels weak

[TouchandFeel]

First off, we should all avoid comparing any of the other tanks' defensive capabilities, especially self-healing, against WAR in the context of large dungeon pulls. WAR is arguably immensely over-powered to the point of being broken in this scenario and does not make for a good point of comparison. Don't point to an outlier when you are trying to make an argument of how something compares to the rest.

For me personally I had no noticeable trouble with doing wall to wall pulls in dungeons while leveling DRK to 90 and for the most part the capability of being able to comfortably do that didn't seem all too different compared to when I leveled PLD to 90.

Going into leveling DRK, I really wanted to see for myself how bad it was defensively in dungeons since that is what some people were saying and so I was very meticulous about my approach; making sure that I always went into each dungeon at min level, always did one initial run using the Trust system to get a consistent base-line of performance per dungeon, and then did at least 4 runs back to back to have some amount of a sample size to average out to compensate for variable party competency.
Only during the largest pulls using a Trust party or if the party finder healer was so bad they could learn a thing or two from Alphinaud did I ever have any issues, and this is no different than when I was leveling PLD.

So the argument that DRK is "fine" in terms of basic defensive competency for large dungeon pulls is true. However, the differences in opinion come in because of differing definitions and expectations for what "fine" means.

Using the context used above to define "fine", that of basic competency in a range of average situations, DRK will perform approximately the same as PLD and GNB, or at least has the potential to.

The caveat to this is that I noticed that in this situation, DRK is a bit more involved or busy when executing it properly. I felt that a bit more thought and consideration was needed when determining how to stack/layer my defensives with TBN and how to best fill the gaps between TBNs. You are also inherently pressing defensive buttons more frequently with TBN having a recast of 15s and you wanting to pretty much always use that as often as you can. With pretty much everything revolving around TBN and how frequently it is available, you end up having to pay more frequent attention to the decisions that you make which can feel frantic and/or frenetic, leading you to very much love or hate the way it feels.
With PLD, defensive usage feels way more just like a cyclical rotation where you are more focused on the bigger longer recast defensives (Rampart, Sentinel, etc.) and then just layering in Holy Sheltron as it's available. It feels incredibly straight forward to the point of almost not needing to think about it as you do it. The only real level of thought you need to do, which is still minimal, is making sure to use your Requiescat + Holy Circle burst during periods of peak damage which always tends to be towards the beginning of a big pull so is really easy to remember and do.

The other side of this disagreement though, is if in your definition of "fine" you are including the ability to adjust to situations that have gone a bit off the rails, such as if you have a really bad healer, the dps keep dying forcing the healer to spend too much time on them, or the healer themselves die. Now while these kind of situations are normally few and far between, they do happen and I ran into instances with both PLD and DRK. The strong self-healing on PLD and especially having Clemency for emergencies gave me more flexibility to deal with these kind of situations, while with DRK I didn't feel quite that level of flexibility to keep myself sustained in a protracted engagement without a functional healer since there isn't as strong or constant a level of self-healing.

If you also factor in the invulnerabilities, yes Living Dead is definitely the worst of the bunch and especially so in party finder dungeon runs where it is can actually be dangerous to use. So the other tanks do have one extra strong defensive tool that they can leverage in dungeons over DRK. I personally feel that all the tanks are defensively strong enough that the invulns aren't even needed, but if you have them and they don't cost anything to use, make use of them.

So in conclusion ...

If your definition of "fine" is that DRK can perform well enough in large dungeon pulls to not be in imminent danger with an averagely competent party, then yes you are correct and DRK is "fine" by those standards and absolutely not the dumpster fire that some are making it out to be.

If your definition of "fine" is that DRK is equitable with the other tanks overall in all defensive aspects and all potential situations, then no DRK is not "fine" by those standards because there are discernible aspects and situations in which DRK lags behind the other tanks and could use some changes or adjustments to address those weak points.

These two things are NOT mutually exclusive and in the end both are true.