I've seen more switch failures than battery failures. Only saw one batt failure airborne 2 while charging.
seen a couple of switch failures.
Bottom line here is redundancy 2 batts 2 switches, is it fool proof no. A shorted battery will bring down the other. A batt share monitors the batteries an isolates one in case of a large amp draw compared to the other batt. If the batt shorts internally will you may have that flying cinder I was talking about