-
The only real use they have is if you are close to respiratory failure; I bought one after I ended in emergency; it can help a little, but not when sleeping. It was kind of backup, if you are travelling, far from emergency services; in that case, it may help. But then, any air pump (even a tyre pump), o diving equipment (air bottle with reductor) can work well, and better than CPAP.
CPAP provides relatively low extra pressure, which can help a little bit, but not enough;
I was mad in emergency room that they told me it was either intubation or nothing. What people with myasthenia really need is relatively high pressure air supply which they can control themselves (i.e. you take a breath through hose, allowing the pressure to fill in your lungs, and then you exalem removing the hose (I got rid of the CPAP mask)). This was at least my case a few years ago.