What would DNC be better at over the other hybrid jobs or DDs?
Dealing very large quantity of damage over a short amount of time (under 1 min).
This is done by preparing before engaging the target and having all the necessary cooldown and resources ready, prepared in advance.
This allow DNC to reach incredibly high burst of damage, supposing the target isn't strong against Piercing damage (or the SC damage, which for DNC can be very various but tipically will be Lv3 Darkness damage).
I guess this is probably the key difference I'd mention compared to the other jobs you mentioned, imagining average gear for all and a low amount of specific gearsets.