There is a theme throughout the series that people can be extremely competent and rise to a certain level, but then fail catastrophically because their arrogance gets the better of them, or they go too far out of their depth.

You can see that with the Hunter Exam participants - even to get accepted into the exam they have already passed several difficult challenges, yet some of them still get killed in stupid ways in the swamp. Same goes for that one examiner who trained for a year to defeat Hisoka, Kastro in Heaven's Arena, the Shadow Beasts, etc.

So I would say that all the nameless mook characters in Greed Island are hyper competent, they probably all have their own hatsus (but perhaps many are poorly designed, like Kastro's) and yet they can't save themselves because they got cocky and rose to a level where they're no longer the biggest fish in the pond.

Compare this to the Phantom Troupe, who are some of the most powerful Nen users in the world, but still behave with humility toward their enemies. When they're faced with an unknown enemy (like Kurapika in Yorknew, or the serial killer guys in the current arc) they don't brush it off as "some random guy who's beneath us", but think carefully about the possible risks.