3. Ease of healing (healer point of view) drk takes considerably more dmg than a pld and slightly more than a warrior (this matters in content that matters).
4. Damage application viability: A drk needs to choose what and when he can contribute some dps, at the cost of losing defensive skills (dA for dmg, or dA for mitigation) where as a warrior doesn't need to choose, he just swings.
5. Concept and lore: darks have never been tanks in any ff game, it breaks the lore, craps on the entire ff history, and shouldn't have been added as a tank.
6. Final thoughts, they are mediocre at best, check reddit, check forums, check yourself, they are mediocre with huge button bloat and terrible resources to manage.