For years the use of full-face snorkel mask have been all too good with all its great benefits that cannot be gotten from the traditional type of equipment or mask, as this mask allows you to breathe both from your nose and mouth underwater so you need not have separate pieces for it.
These full face snorkel masks are good in a sense that you will be more relaxed with one of these on, rather than constantly clenching your mouth, they are less intrusive, more hygienic, 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.
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.
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.