On the sending side, there are two catches: you need to know which gateway to connect to (which in turn generally costs money), and your messages look even sketchier than "normal" short-number SMSes.
On the receiving side: there isn't any real delivery guarantee for SMS/MMS gateways. You're basically depending on a mostly forgotten and probably neglected service (and probably physical server) at each carrier to receive potentially important information.
Edit: You can also get around the gateway identification problem by spamming every gateway with your message. That worked pretty well when I tried it last (around 5 years ago). But I wouldn't be surprised if continually doing that earns you a hellban from every carrier.
You have be in the US. These email to SMS gateways all shutdown 15 years ago in other countries. I suspect the US keep them because providers handle billing differently. Unlimited SMS as been include in most subscriptions for more than a decade here, but some Americans still pay to receive texts. So in the US there’s still a finacial motivation to keep them. That and the US seems to be rather slow to move of certain outdated solution.
