Feeling squishy as a DRK could be from a few things such as low gear item level on either you or the healer, or it could be the healer is focusing at dps and reacting to your healthbar when it gets a bit lower.

The only thing honestly wrong with DRK is the fact that when it comes to heavy partywide damage, they aren't able to mitigate them like Divine Veil or Shake It Off can. they only have Reprisal which any tank can do.

As far as damage, they're honestly fine and are actually coming close to WAR in damage if you look at the top damage dealing tanks for 4.4 Alphascape raid tier.

Gameplay wise, they can feel a bit clunky with the heavy reliance of needing to excessively use Dark Arts to buff the next skill, but during Blood Weapon can be difficult to double weave such as Dark Arts+Carve and Spit or Dark Arts+Dark Passenger. Then the mini game of managing Mana can be a bit daunting while trying to optimize yourself.