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.
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.
With the raising concerns about full face snorkel mask, there is a need to ensure that such mask is used accordingly as to its purpose or as to what it is intended for, that is 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.
So then, when you choose to purchase full face snorkel mask be sure to try on the mask and breath in it on land, and make sure only the breathing part should fog up when you try to breathe on it, and bear in mind the purpose of buying it that it is intended for relaxed snorkeling only and not made for freediving or extensive open water swimming.
One important thing that you will also remember when purchasing a full face snorkel mask in order for you not to encounter any issues or problems when using it, is to make sure that it fits well about your face and that its breathing part also fits neatly over your mouth and nose.
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.
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.