Hangouts can send and receive SMS at the moment, but I don't have any idea when it might be shut off. Hangouts is being split off into just two functions: Hangouts Meet (like multi-party video conferencing) and Hangouts Chat (like any traditional instant messaging service). The other functions have migrated to Google Voice, which can send and receive SMS/MMS and (for beta users at the moment, soon to be generally available) make and receive GV calls over VoIP.
So, don't use Hangouts any more. Just use GV.