SMS is not guaranteed to deliver, either within a certain time boundary or even at all. I can only really speak from my own limited experience on AT&T and Verizon, but those delays seem to largely be gone. I haven't had a message delay in quite a long time. If it's going to deliver at all, it'll deliver within seconds.
Yeah, I imagine the technology should have improved from a few years ago when the delays seemed prevalent. You're also right that some messages may not go through at all. I'll probably need to research deeper into the current technology and issues surrounding SMS.
Anecdotal, but I have experienced an SMS message delivery delay of about 7 hours within the past week (on Verizon). I would agree that delays are less common, but they're by no means over with.
