It's more the breathing issues, eye and dental problems and the fact that they are very heat sensitive. Here's a link describing some of the problems.
Brachycephalic
I think the inability to deliver in some breeds is more due to the proportional size of the head rather than the shape.
You can look on the internet and browse some of these breeds. Take a look at old drawings and photos from a century ago and then modern photos. While some of these breeds are quite old, it's the modern standard that has pushed the dogs' features to the extreme, creating the problems. That's where good stewardship of a breed comes into play IMO.