I'll lead by saying I'm not a DRK main, I play every job somewhat equally (ADD brain), and am decent at most (imo), not really great at any.
I enjoy tanking, so I tend to play that role most (somewhat for faster queues tbh), and while I love the job fantasy of DRK, I struggle in dungeons with it the most of all tanks, and struggle healing DRKs the most of all tanks when playing as a healer. This is almost exclusively pertaining to dungeon pulls, as I feel like DRK is on very even playing field (or even above) with the other tanks in a boss fight.
I enjoy playing PLD the most, as I am the most comfortable with its kit. But sometimes I like going ham on some DRK.... although I feel it really lags behind the other tanks in survivability, something that I somewhat felt GNB suffered from as well before Heart of Corundum. So most of the time if I feel like going with another tank than PLD, I'll go with GNB for dungeons, DRK for anything else.
What I think would fix this issue is to add a heal to either Edge of Shadow and Flood of Shadow, or to Stalwart Soul (but obvs a smaller heal in this case). Larger heals if put on the oGCD's (since they're limited to MP usage) than Stalwart Soul, I think. At least this way, DRK would have comparable survivability to GNB in dungeon pulls.
From a lore perspective (at least from what I understand of DRK in other FF's - I've only played FF4, and as the Dark Knight portion of that game was so short, I don't remember much specifics of what its abilities were like) it seems odd to me that DRK doesn't have more lifesteal in its abilities, as what I remember of the job in FF4 was "hurt myself to do more damage, and lifesteal to heal back up"... although I may have that partially (or totally) wrong lol.
Or just, take 10% of the crap they buff WAR with every other patch and give it to poor DRK. >
TL;DR : Give DRK a heal on Edge/Flood of Shadow or Stalwart Soul to give it better survivability in dungeon pulls, like the other tanks that don't get neglected by the Devs.