bad most of the time they will make a fake cave and trap innocent inverts like hermits and eat them but they are great scavengers i have them in my reef 75 gal so far no issues that have been SEEN
large, large, LARGE brittles may catch fish or inverts if they can grab a hold of them. But just about all healthy fish are strong enough to get away as long as the star isn't ginormous. But honestly, i truly believe that almost all of these reports of them eating things are just scape-goats for why a fish died.
Also, they dont have a brain, they cant build a cave to trap something. But may try to suck on to something that they come across, but once again, that pretty much never successful.
The mini-brittles are perfectly fine and are a good part of your clean up crew. You probably have a few in your main tank too.