Fortunately I have never needed to use these systems in anger, but I have worn a rebreather for some time as a stand alone device. I have measured oxygen and carbon dioxide levels in a hospital setting using the device, and even with maximum hyperventilation I don't get uncomfortable hypercarbia for well over 90 seconds, and beyond this it is rather academic. The size of the pocket is large enough so that one exhaled breath - in practice little over a litre - does not distend the pocket to prevent egress.
A significant benefit is that it can be deployed during the descent so that on contact with water all you have to do is pull the pin on the mouthpiece. The mouthpiece is in your mouth and locatable. By comparison any compressed air system has to be taken out of a pocket with a risk of dropping it especially in cold water. Then it has to be rotated, possibly in the dark, so the mouthpiece faces you, and then you have to expel water from the system.
I am sure with practice this is all possible, but just replacing a mouthpiece underwater often causes difficulties for amateur divers.