I don't really tank, but I do heal a lot. I can tell you that a DRK who knows what they're doing will seemingly be taking nearly no damage save for the starts of big pulls. You can slap two or three OGCDs on them and they're golden (compared to WAR who doesn't need you at all). They're fine. It's just that DRK is on the more complex side of things when it comes to actually playing it well, I guess.