"Have yet to read a negative review on one" translates to "have never read anything meaningful on them".
All that you have on the internet is bros buying them, noticing that training with them on is really fucking hard, and then recommending other bros try it because it makes you tired (obviously the best way to measure how effective a training modality is, right?).
There was an excerpt testing results of a person who used one for 8 weeks in a thread a while back- basically, you saw almost no positive training adaptations from them.
There are a shitload of threads on this site about training masks, and nothing positive about them, except for that Sebastian dude who appears to be the ultimate king of the bros (even more than belph).