Hello! I would like to start this off by saying that I am posting this as a fairly casual player who spams a lot of dungeons. My experience comes from tanking dungeons, and I acknowledge that the DRK job may feel entirely different in content such as savage raids.
Dark Knight was my last tank to level up in Endwalker, and I have to say, it was a dreadful experience, because of the DRK mitigation. In comparison to every other tank job, it's terrible and lacking. Despite proactively weaving my defensives on a DRK, I often run into situations when I just run out of good defensives. This is especially painful when you're pulling wall to wall, but even when doing small pulls it can hurt sometimes.
On PLD, WAR and GNB I often make pulls where I hit a good stride with my defensives and it feels great, I feel like I'm taking very little damage, I see that the healers don't need to spend GCDs on spot heals, everything is great. On DRK? Oh boy do I see healers whip out those GCD heals, and it hurts to see.
I don't think I can exactly name the issues. The Blackest Night feels like a joke compared to the short-CD defensives of other tanks. Also, for some reason, DRK is the only tank where your defensive ability shares a resource with your offensive abilities. During big pulls, The Blackest Night protects me for about 1 or 2 seconds, while the other tanks, with their short CD defensives, can enjoy protection for 8 wonderful seconds.
Living Dead is an absolute joke. The invulns that revolve around you surviving at 1 hp are already all useless for dungeon tanking, but this is the only one that can kill you, and that the healer can just end early by being a good healer. I only push Living Dead on accident and it always feels bad, this ability is terrible.
I would also very much like if on tank jobs, there was a little more focus on defensive utility even in their damage rotations and such. You know how all the tanks have a hit in their combo that does a little self-heal and such? Yeah, that's great, more of that please. There is one tank job that does this well and it's paladin. Most of the damage abilities you use as Paladin have defensive utility and feel great to use. I guess this is not exclusively a DRK issue but on GNB and WAR the mitigation is flawless, so there's nothing to complain about.
It feels terrible when the job design is focused around me keeping up some lousy DAMAGE BUFF while juggling blood gauge which only helps me do DAMAGE while I am GETTING KILLED. As a tank, I want to not die FIRST, do damage SECOND.