Full face snorkel mask had been well recommended for snorkeling and the best thing about the use of this mask is that it allows you to breathe both from your nose and mouth underwater so you need not have separate pieces for it.
These types of snorkel 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.
The concerns that are raised with the use of full face snorkel mask are due to the misuse of such equipment when it is supposed to be used according to its intended purpose such as 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.
You should never forget to always go for the best quality and brand of a full face snorkel mask and never settle for cheaper/knockoff models and imitations that are widely available in the market as that kind of brands have the most safety-compromising result.
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.