I have caught a few swarms from mini-nucs where the queen has already been clipped. Yes, I know I should have put an excluder over the entrance at the same time! However sometimes you don't get there in time after mating to put an excluder over and she's off literally before you know it!

Anecdotal evidence from me, in that I have never counted, is that mini-nucs are less reliable compared to larger nucs. Here are some reasons, you may have more:-

Colonies can get too big very easily.
Use up all the food.
Absconding.
Laying workers if the first queen or queencell fails. (With no brood present).
Too weak/small to defend wasps.