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.
The very common concern with full face snorkel mask is the CO2 build up, but the manufacturers now have already addressed this concern and are able to produce innovative solutions to make this equipment more reliable underwater with all its safety.
So when you choose a full face snorkel mask, remember that it is intended for relaxed snorkeling only and not made for freediving or extensive open water swimming and to get the best quality you need to try on the mask and breath in it on land, and make sure only the breathing part should fog up.
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.
The quality of the full face snorkel mask must be the top one and you have to be very wary of those that are out in the market, even on known online websites that are cheaper/knockoff models and imitations that have been the most cause of many safety-compromising issues about full face snorkel mask.
It would be preferable that you make a good research and gather the most detailed info about the features of the mask, where it came from or manufactured and do not settle quality over the price, it would even be more advisable to purchase those expensive types especially if you are going to use this often as you can guarantee about its safety too.