The use of full face snorkel mask had been popular even before and they are best recommended for snorkeling, and what is best with this mask is that it allows you to breathe both from your nose and mouth underwater, not needed any separate ones.
The best benefit that you can get in using these full face snorkel mask is that they are less intrusive, more hygienic, and you will be more relaxed with one of these on, rather than constantly clenching your mouth, and they are a bit less constrictive for people with respiratory issues.
Despite the issues that concern the use of full-face snorkel mask, manufacturers have ensured that such masks are being used according to its intended purpose to alleviate the problems that are caused by its misuse, that it should only be for recreational use or just floating on the surface and comfortably looking down onto the fishies.
One of the issues that also raised concern with the use of full face snorkel mask is the CO2 buildup from breathing, but then the reputable manufacturers of these masks have already made innovative designs on it in order to address the issue and to make the equipment more efficient and reliable underwater that does not compromise safety.
Therefore, when you are choosing to purchase a full face snorkel mask always remember that it is intended in general for relaxed snorkeling only and not made for freediving or extensive open water swimming, and to get the best choice try on the mask and breath in it on land, and make sure only the breathing part should fog up when you breath.
Also make sure the mask has the correct and exact fit over and about your face with its breathing part fits neatly over your mouth and nose, because if all these fails then you will have trouble or problems in using the full face snorkel mask.
Also it is important that you check the quality and brand of the full face snorkel mask, and be wary of the cheaper/knockoff models and imitations that are out in the market as that has the possibility of compromising your safety when using the mask.
Nothing will beat an informed purchaser, and it is more advisable that you research about the product that you will be purchasing especially from where it came from and manufactured, and all the same get as much info about its features so you can have all that you need to know, above all never compromise the quality over the price as it will be more convenient to buy an expensive item where you are assured of quality and its safety hen you use it according to its purpose.