I own one myself and I have never thought about the build up on CO2 that would be harmful to me. I must say that I have much more enjoyed my snorkeling adventures since I own this mask. I have done many hours on the mask and no fog at anytime which is a big improvement for me. You do need to make sure you don't clog up the mask when you don't have the proper position. The breathing is something I haven't thought about being the part that could make it dangerous.
I wonder always out of the those stats how many people were beginners? They have become confident and adventures in areas they may have not? I'm not a confident swimmer so for this reason I always were my own lifevest and try to stick to distance I most confident with. My partner is very confident in the water but take this in consideration to not place both of us in distressing situations.
Saying that being in the water always has its risk as you are just at tiny fish in the ocean