I cannot really think of a clear reason for a loss of consciousness, having used FFMs at their maximum rated depths, for years, save for some unforeseen medical issue; or perhaps some ill-advised gas mixture, though most of us were typically diving on air back then. Positive-pressure masks are more efficient at venting any potential CO2 accumulation.
“So you lose consciousness in the FFM now what's supposed to happen that the FFM solves?”
Less of a chance of drowning under certain circumstances?
In an earlier thread, I had mentioned that we came upon a fellow diver who had suffered what later turned out to be a mild epileptic seizure; and had he not been using a FFM, would likely have drowned with the use of a conventional regulator -- more than likely losing his mouthpiece.
He was still regularly breathing, when he hit the surface, and that FFM probably saved his bacon . .