ResMed AirSense user here. Do take note that the water reservoir in the recent versions is prone to leakage after around a year of use, and replacing it is pretty expensive.
I sealed mine with food-grade silicone as fellow users in the internet suggested, and it's been good since.
The water tank for the AirSense 11 is something like $29 and it's a consumable item that's covered by your insurance. I just get a new one every six months.
I don't know - I guess because you could get build-ups of minerals and other things inside them and it's probably better not to rely on people cleaning them given that they're medical devices that blow air into your lungs?
I sealed mine with food-grade silicone as fellow users in the internet suggested, and it's been good since.