DOFF system - Getting a critical success used to give 500 or 1000 dilithium. I'm not sure if that's been nerfed yet. Normal payout is 5 dilithium. There are three things that you want to look at when assigning DOFFs to an assignment, their rarity, their specialty and their traits. Most players favor stacking for a critical success. Look for the DOFFs that increase both the success and crit success rates. You can experiment by slotting a DOFF then highlighting another one on in the roster and clicking "swap...". XP for the DOFF assignments has been nerfed quite a bit since inception. According to Heretic, it's been nerfed 5 times. I think that they should take out the XP rewards and just give appropiate commendation points. Of course I can say that since most of my characters have already benefited from the generous XP payouts. :-P
You don't have to experiment anymore; the DOFFs now highlight any skill they have that contributes in green, so you can see at a glance who's got what.
Also, you can filter by required or excluded attributes if it helps. Filtering out the ones with the attributes that contribute to Disaster is always helpful.