I have the ecobee 3 lite and I try to keep it off the cloud and connect through home kit to home assistant. It is exceptionally unreliable and disconnects all the time. It's the only home kit device I have, everything else is zigbee. I don't know if it's an ecobee issue or a home assistant issue or maybe a router issue. The only thing that's helped some is occasionally letting it have access to the cloud and then it will reconnect to home kit and then I can disconnect it from the cloud and it will be good for any more from a few days to a month. I cannot recommend it because it has been so unreliable, and looking through online forums I've seen a lot of other people have similar issues with HomeKit and the ecobee. Eventually I want to switch to a purely zigbee device, as this is the only thing I have that even has Wi-Fi on my automation network.

I then control temperatures through a combination of home assistant, node red, and sensors through zigbee2mqtt.