Just installed in garage-separated from house with enclosed breezeway and required metal door. It's Matter over Thread.
I specified a HomePod mini in the kitchen as the hub of choice (it is closest of the three I have).
No other thread devices, ie, repeaters.Not running any other hubs at the moment, so observations are via Apple Home Alone (lol).
I understand a HE driver is available.
-Connected to Home very easily
-Weak, like Zigbee, but seems to work now where I have it located in garage, along with the HomePod hub's location
-Has a digital readout for CO
-Doesn't notify when tested
-Does indeed send notifications when a smoking paper towel was waved underneath, lol
-Sent CO as well as Smoke notifications during test
-Uses a CR123A battery, supposedly good for 3 years
-No notification when it goes offline-have to go to Home app
-Euro standard. Supposedly US version is in works. I didn't notice any difference with the way my US devices work.
Only having a HomePod as a hub, I have to find a way to sound the alarm in the house. I don't see an Apple Music subscription in my future or a matter siren so I don't know. Perhaps have a noisemaker powered by a smart plug. But that should not be a problem for the Hubitat user, assuming the driver works. 
