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.
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 most common concern with full face snorkel mask is the CO2 build up, however seeing this concern already have made the manufacturers create innovations to ensure that this is addressed to the benefit of the user so that it can altogether more meet its intended purpose with all the 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.
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.