While these end up as SMS notifications at your phone, you've built a Rube Goldberg machine that depends upon processing in the SmartThings Cloud (which has been highly unreliable in the past) and a whole bunch of data connections in between. There are lots of places for delays or lost messages. Using HE Push Notifications is an order of magnitude simplier and more reliable.
If you really want free (or near-free) SMS messages, you might want to look here: [Release] Textbelt SMS Notification Driver. It uses an external service to initiate the SMS, but directly via an API.