Problems with DRK are numerous. Living Dead needs tank/healer coordination, DRK has basically no utility as OT and always wants to MT, only 7 out of 22 skills use Dark Arts.

Only Souleater has a Grit effect.

And then you have Parry. Which only matters in MT role. And can still suck.

DRK is okay but definitely could have been done better. Especially when it's just a mish mash of concepts pulled from PLD and WAR making the DRK at times have a lack of identity.