[woodruffw]

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.