So there are two possible ways you can send bitcoin through an SMS message.

The first way is when you sign a transaction form your own wallet, and use an SMS message to broadcast that signed message to the network. This way is completely safe, provided you use it correctly. Once you have signed a transaction on your own wallet, it cannot be changed or altered in any way without being made invalid. You need to trust the third party service or person to actually broadcast your transaction that you sent them in an SMS, but the most they could do is refuse to broadcast it, meaning your bitcoin simply stay in your own wallet. They are unable to modify your transaction to steal your bitcoin, or anything of that nature.

The second method, which is the method being used by the service being discussed in this thread, is when you are trusting a third party to hold all your coins on your behalf, and you are simply using SMS messages to send instructions about what you would like to do with your account. In addition to all the usual security risks of storing your coins with a third party, you are also at risk of someone impersonating you by either physically or electronically taking control of your phone and sending SMS instructions on your behalf, SIM jacking your phone number, or even linking their own phone number to your account. In all cases, someone could send SMS instructions to the third party to transfer all your coins out to the attacker's address.

As with all things in crypto, storing your coins with a third party is inherently risky, and this is no different.