DL has only really made one, maybe two draft errors.
Trading the #12 pick in 2016 for George Hill was definitely an error. It was an error at the time and it is an error in retrospect. Sure, the season afterward we went to the playoffs. But I would argue that we were well on our way to making it anyway. We were still in the rebuilding phase at the time and trading a lottery pick/cheap scale contract for a one year rental was silly. I said it way back when the trade went down...If you don't like a player in the 2016 class trade the pick for a future pick(s).
The Dante Exum pick is looking like a possible error too.
Other than that DL has done a solid job. People can complain about not drafting Giannis, Booker, etc but fact is EVERY GM has misses like that. The deeper into a draft you go the more of a crapshoot it becomes. We can call Dante a possible mistake because he was drafted at 5. But later "misses"... eh. I can't get on him about that. Also, I personally dislike the approach that they've taken toward second round picks but I understand the approach. I'm not gonna bash him over that.