I have Rule (v5) that if ANY Thermostat Operating State changes it should run.
Unfortunately the RULE frequently fails to run when a Thermostat changes its Operating State so have had to add a timer to the Trigger.
Any suggestions on how to get the Rule to work reliably without a failover periodic poll?
Can you post a copy of the rule, and does logging reveal any useful information?
I'd find evidence for this claim. 
The fact that your rule isn't having the intended effect, which I assume is what you are noticing, doesn't mean it isn't running.
Turn on trigger and action logging for the rule (or however much more you want) and see what is really happening when. It is possible that on1() command is getting sent to your device and it's simply not responding, or perhaps it's not correctly reporting its state and is getting skipped per your conditional. Something like this is often the case rather than the rule itself not triggering.
Are these thermostats zigbee/zwave or wifi?
The Thermostats are Hubitat Apps - Thermostat Controller
Each Thermostat Controller - manages a room with a Temp Sensor (well two per room one zigbee, one zwave) and a zwave Thermostatic Radiator Valve.
You raise a good point!!!
I'll take off the periodic trigger.
I'll add in send a message to my Phone Stating that the Rule has Run with date/time stamp
I did have Logging for Events/Tiggers/Actions enabled but there is nothing in the EVENT log for this Rule.
Is there something else I need to turn on to get the Rule to log actions?
I don't know what event logging is supossed to do (there is a good guess!), but trigger and actions logging will show up under the reguar "Logs," so that's where I'd look after it's supposed to have triggered.
Ha
In my addled brain I had expected the Rule Events to be in the Rule Events tab, my bad 
Will check out logs
Download the Hubitat app
