I often allow my players to decide what happens, vetoing anything that seems to powerful or that dosent fit the action that was being performed by the NPC. It allows them to choose if they want something that gives them a slight benefit, or something they think will just be funny/interesting. 99% of the time players wont choose something that’s over the top, most pick something like dropping a weapon in combat, or slipping up with some minor info in social intrigue, or a minor thing that sets up something interesting for them (had one where player was fighting an NPC and I rolled a complication, they were on an airship so said the complication was the NPC stepping backwards putting his foot in the center of a rope coil. Set it up so next turn the PC could try and pull the rope coil tight to restrain the NPC).
Sometimes if its towards the end of a longer combat or one that’s gone on for a little too long, I’ll let it be a ‘NPC makes a fatal error’ kinda of thing.
Unless I’m low on threat or really want to show the PC’s that the situation is dangerous, I usually don’t add Threat to the pool, but that’s personal preference.