I went with the Apple HomeKit compatible camp, because it does indeed all work locally across your local LAN without a network connection - its not foolproof, but it mostly just works, and works well at that - and more importantly works when the internet is out. The biggest thing with Apple is they get that houses have more than one person living in them, and provided everyone has an Apple device, it all just works.
Our place came with an Ecobee thermometer. I threw it up on Homekit and my partner had it on their watch and phone instantly. Too easy.