I've seen them and I believe they're inherently more dangerous.
My wife wanted to get one. She is in her 50s and although she wades and floats in the water she was never comfortable getting her face wet. Weak swimmer. Typical of someone who buys a ffs. I encouraged her to get a regular mask and snorkel, and she did. And worked through her fears in the pool. Sometimes by herself (with a lifeguard), sometimes with me. She worked up to snorkeling in Key Largo at the Christ of the Abyss statute, with me, while the kids were scuba diving. She had a fantastic time.
Standard snorkel, user panics, they spit out the snorkel, stand up (or swim), and breathe air. And orally inflate their snorkel vest if they have one. (My wife did but never had to use it)
Ffs they have to use their hands to take it off, costs seconds, maybe tens of seconds, can't inflate until it's off, can't breathe until it's partway off. The difference between a bad experience at the beach and a fatality is measured in seconds. How can anyone maintain that there is zero added risk?
What if the chin cup doesn't seal or the inhalation valves don't close? That's where you get the CO2 buildup. Would you realize it, in warm water where there's no condensation? Would someone who hasn't snorkeled before realize it? Another risk
If ffss have added risk in those situations the only way they can be as safe overall as a regular snorkel is if they are somehow safer in some other area. I don't see where that could be the case.
Why do we have to wait for a peer-reviewed study showing a statistically significant relationship before anything changes? Why isn't the onus on the product manufacturers to demonstrate that their innovation is safe?
And in the words of Bob Dylan, How many deaths will it take until we know that too many people have died?