Whilst I admit I don't have the vast experience and knowledge others do here, the only instances I've heard of of
furling lines letting go has been a result of either chafe or (more usually) slipping off the
cleat.
I've never heard of a
line snapping from sheer force and can't see how the usual set-up could be loaded enough for it to snap.
Aside from it coming off the
cleat (usually a v-jamb or similar), chafe can happen anywhere depending on how the
line has been run. I've not heard of chafe happening at the
cleat or on the drum, but I've heard of a
jib lazy sheet cutting through a
furling line run along the deck and over the cabin-top in a rather haphazard fashion.
I've seen people run
furling lines below deck to avoid chafe..
