My 2 cents on this.

DRK is trash in dungeons, for 2 reasons. LD is something you have to tell the group you are using, or your healer will legit wonder why you're afk. You have to use all your buttons NON-stop to feel like half the tank the other 3 are.

PLD on the other hand. I could remove every mitigation other then holy power shield thingy and my normal rotation will easily keep me alive and easy to heal. God forbid I use rampart and sentinel the healer may as well afk.

The BIGGEST difference I have between DRK and other tanks (maybe not GNB, I dunno I don't play it)

Is I cannot carry a boss fight and heal my party, at all. Ever. When the healer goes down, so do we all. As PLD I'm legit 75% of a healer, and I even have multiple party mitigations to supplement it.

Why would I play DRK in a dungeon then, it is by far my favorite class to play, and the game makes me feel bad for playing it. I can't just go in blind on lower end content like alliance raids or trials. But on PLD I can, and not only live but often Main Tank the fight knowing nothing about it. Its just so much easier and safer to play.