I've noticed that two quests involving killing some spawned bandits are problematic.
First is where a guild master asks you to deal with some looters around the city. First of all those looters immediately flee from you so you chase them across half the map. Second, local armies attack them as well, and if someone else attacks and defeats a looter band, you fail the quest (or rather it becomes uncompletable), even if you joined the battle to help the 5 scouts that attacked the looters first. Additionally, sometimes I myself defeated all the looter bands that I saw and the quest still did not complete - do some of them spawn too far perhaps, outside spotting range?
The second quest is also from guild masters who ask you to deal with troublesome bandits. It has all the same issues, though you immediately fail the quest if someone else attacks those bandits first, even if you helped and had the biggest force. Some more robust checks need to be added to those quests so that at least if you have the biggest army, you are considered to have defeated them even if some piddly group of 5 scouts attacked them first.
Better yet, make them ignored by normal NPC armies, like those bandits that kidnap a girl.