Tank balance is fantastic in FFXIV and anyone who says otherwise is a 0.1% speedrunner using speedrun strats and cheesing stuff. That said, I always break it down into two categories:
DRK is great at clearing content; its only weak link is Living Dead, which can be overcome by playing smartly in a static but that's a stupid high bar for something that could be a class-defining ability. Even despite Living Dead, DRK is arguably the best tank for prog and won't hurt your farming.
That said, DRK also plays like ass. Compared to how fun Warrior is and how smooth Paladin plays, DRK is just needlessly awkward due to Dark Arts. The trouble is they did a FANTASTIC job with the other two tanks and DRK just doesn't live up to that incredibly high standard.