I've had three batteries swell on me. The swelling wasn't enough to cause unlatching. The worst of the three had a weak latch to begin with, so it could unlatch in flight. I pulled that one apart to where I broke the latch sensors so that one won't fly.
Anyway, I tested under controlled flight (close by hover low to the ground) and flight time diminished only by a minute or two compared to a new battery.
My conclusion is that the main issue is with the physical issues from the swelling where the batteries are otherwise just fine. They won't likely burst into flames. Anyway, if it were to become unstable, it would more likely burn up during charging than during use.
The main issue is with becoming unlatched and falling out, or the battery losing electrical contact with the AC.
It is only the center two pins that are for communication. The pins on either side of the two center ones are for power and are redundant, the redundancy for more current and less contact resistance.
Straps may prevent the battery from falling out, but there's still a chance that the AC lose electrical contact with the battery.