Everything I have heard about it is that it is insanely complicated. You have to deal with different radio frequencies and communicating with the right one properly and reliably. I think that Apple thought that since they did an excellent job designing their own chips, that modems would be a cinch but this has proven to not be the case at all.

Man, the betas for iOS 11 were so buggy that I couldn’t even count them all. It only got good about halfway through the beta cycle. This years and last years beta cycle gave me no problems in contrast.