I really can't imagine suffocating in a bivy, even when zipped tight, they are nowhere near airtight. About the only danger I can imagine is sleeping in one during a snowstorm, if covered in snow it can become a serious problem, but the same danger exist in a snow covered tent.

I have one of the goretex bivies, which is basically the same concept, and I have to say my limited experience with them isn't exactly favorable. I admit that I was influenced by the romanticized notion of having a high tech, self contained piece of equipment, that I could just climb into and ride out any weather, but the reality of it is pretty different. I think they might be great for their intended use, which was emergency shelters for mountaineers who get caught in a storm, but as a primary or even backup shelter I don't find them very practical. It offers no protection for your gear, it feels very claustrophobic, and if you get wet everything inside will be wet also. You can alleviate some of the problems by rigging up a tarp to shelter you from the rain, a mosquito net to protect you from bugs, and a ground cloth to isolate you from the ground, but by the time you carry all that plus the bivy, you might as well be using a lightweight tent. This is an opinion from someone that really wanted to like the bivy, but just couldn't overcome it's limitations.

For a stricly emergency shelter, I prefer to carry a large garbage bag or lightweight silnylon tarp. It's smaller, lighter, and more versatile than a bivy. A cheap, rectangular piece of plastic just isn't a cool as a goretex cocoon, but sometimes simpler is better. I can never find a good reason to carry a bivy, it's too heavy to carry just for emergencies, and if there is a very strong chance that I would need to spend the night out in the kind of weather that would require something as heavy duty as a bivy, I should be carrying a tent.