STASS or HEEDS is effectively the same thing, and has been directly responsible for saving a number of lives post ditching. It's great advantage over the rebreather systems is that you don't need to actually have it in your mouth when you ditch, most of the rebreather systems cannot be "purged" of water in the mouthpiece, particularly if you are inverted.
An unexpected ditching offers little time to prepare youself and your nearest exit may not be available. The breath hold time in water at 10 degrees is around 20 seconds, not allowing for the cold shock reaction on immersion. Yes there is a small risk of air embolism if you don't adhere to the training that is beaten into you. Remember though, you have to survive to run that risk!