The Harmony Companion with Hub will control your RF only Rokus just fine using WiFi Direct which is what the Roku remote for those devices use.
What Is Wi-Fi Direct, and How Does It Work?
Many of the handheld Harmony remotes, such as the 650, 665 and 700 do not support WiFi Direct.
Thanks. This is good to know in case I start leaning towards a solution that requires it.