I think DRK is fine now. You have to remember that TBN gets stronger over the course of an expansion because as tanks our health pool increases more than other jobs. Any additional healing we get would require a damage nerf. If AD cost a large amount of mana instead of on a 60s cooldown it would solve the dungeon issues.
The problem DRK faces is in lower level dungeons before you get TBN. Thats where the job fails as a tank because it was never readjusted after the change to AD back in early Stormblood.