I have been reading and posting in threads on this forum for the past few months and I would say people saying DRK is "unplayable, useless, etc. " is not language players use very often by the DRK playerbase. No one is arguing that DRK is incapable, the loudness comes from the injustice of being dead last in every measurable aspect of tanking, being DPS, mitigation, and party utility.

Just look at say, Shinryu EX, DRKs in this trial using their invulnerability need to be watched by their healers, or else they die. This puts strain on the party, because if others in a PUG group don't know mechanics healers can be preoccupied healing the group. Neither WAR nor PLD need to be baby sat in this way. DRK also lacks an anti-knockback for this fight, again not a big deal, but an inconvenience. Couple this with the fact that you are bringing less damage and no utility, and it's almost a chore to bring DRK, not only for the player but for the healers as well.