Logitech Harmony already has official integration in both directions for the U.S… You can add SmartThings devices to harmony activities through the harmony app, and you can have harmony activities triggered from SmartThings using a combination of two officially-provided smartapps: “Logitech Harmony connect” and “Logitech Harmony triggers.”
See the following FAQ for details:
If you’re in the UK, you may run into the same authorisation issue that is affecting the IFTTT integration, but they’ve said they’re working on a fix for that.