A few years ago I worked for a company which was sending many hundreds of thousands of Twilio SMS messages and observed a lot of undefined behavior at scale. Sometimes messages would be delivered twice, sometimes messages would take hours (or days!) to send, and sometimes messages weren't sent at all (even though we were charged for them). I received all sorts of "explanations" most of them naming various intermediaries in the pipe using opaque black lists and undisclosed rate limiting.

Is the SMS backbone in the US reliable during peak traffic (such as at a large sporting event or in an emergency situation) such that I can trust it as a single-point-of-failure for critical applications, and how does the US SMS backbone compare to that of other countries with less investment in their IT infrastructure? Finally, how vulnerable is SMS messaging to snooping by authoritarian governments compared to other forms of authentication?

FWIW this is definitely a project i'll be following, from the perspective that authentication can often be a "bounce-point" for new users of consumer applications and superior authentication strategies can translate directly to $ales.