The full face snorkels have three problems
1. There's no way to plug
your nose to clear your ears when you're free diving
2. Even if you could clear your ears, when you get down 4-5' the pressure from the mask on your face is fairly unbearable
3. The lifeguards on Oahu
started keeping track of the gear
the people they were rescuing were using and began flagging people with full face snorkels. Turns out that the full face snorkels don't have too large of a chamber to clear with a single
breath and tend to build up CO2. The harder someone swims/breathes, the more CO2 builds up. This was causing a lot of real fit people to black out and drown/almost drown.
I ran snorkel sails
off Waikiki for 3 years and would always tell my guests to be careful and remove their full face masks immediately if they were feeling light headed. The only really good use for these setups is for people who are claustrophobic or can't breath out their mouth.