Devices looking for wireless network restricted to 2.4GHz

many of them are going to say that because customers don't know how to set them up, and they don't want to deal with having to help people get them setup. 

I'm running an ecobee right now on an orbi setup. Works fine. Setup went fine. 

There's a few work arounds to get them running sometimes but it usually isn't to much of an issue. 

I can't attest to your "smart line-voltage thermostat" . 

Worst case scenario is buying a super cheap 2.4ghz router and run it in AP mode for devices that have issues.  You could either just run it all the time or you can use it for the setup process. I've actually done that when Orbi first came out and was having major issues with this. I had the cheap router setup with same ssid and password. I'd get the device all setup, turn off el-cheapo router, and then turn my orbi back on and it'd link to the orbi. i haven't needed to do that in a long time but it worked when the Orbi was still pretty beta.