Yeah, the figure eight/biner might get caught on stuff. But putting a 5 mil line through your belay device is also not really going to work, right? It seems like either way you're going to have to rap off a knot jammed in the chains. I think the eight is less likely to get caught in a link than a double fish. And that is putting aside the question of whether a 10 mil/5 mil double fish is trustworthy.