I can't really say from experience firsthand, but as an opponent, I found one DK in particular to be verreh good.
Strengths would definitely be dmg output. So high *Q*... the variety of weps the DK can multi proc from, and the defense a DK can obtain (though with all the def debuffers at endgame, this sometimes accounts for nil).
I don't know about weaknesses, it depends on the opponent I guess. High evade and anti stun can be a pain for a DK. But not excessively so, or so I've been told. Or maybe I was told that cause it was directed to me. Idk.