In reply to Ghastly Rubberfeet:
"If you breathe in used air full of CO2 leads to headache, dizziness, and unconsciousness. CO2 can build up in wrong-designed normal snorkel tubes too…"
Fortunately, our lungs are very sensitive to CO2. In fact, that's what makes us want to breathe in; it's not lack of oxygen, it's the partial pressure of CO2.
So if you were rebreathing CO2, you would soon get very uncomfortable, and want to breathe in 'properly', long before it posed a risk of passing out.
Regarding the Decathlon full-face masks, that page seems very complimentary:
"Most reliable full face snorkel masks
Easybreath Tribord/Subea
Decathlon was the pioneer on the full face mask market inventing their patented design with engineers and watersport experts. They invested a lot of money and time to develop their products and they still do. They come up with product upgrades, providing warranty, replacement parts, and great customer service."
But, after the CO2 comment (which I fear they have confused with CO, which we don't detect, and leads to the symptoms described), I'm inclined to ignore anything else on that page...