Make sure any thermostat you get has an easy way to clean it out. I know my house thermostats can get clogged up with house dust and malfunction. All the more I would be concerned about using any thermostat with chicken dander in the air.

The older I get, the more I try to make things more simple. When I built my coop, I joined BYC forums and read, and read, and asked questions for almost a year before I built my coop. I learned a lot from the community and there is very little I would change in my build if I rebuilt it today. So that was a success, But I live in the great north where our temps get to -45F in mid-winter. My concerns were always on how cold the coop would get, never on how hot it might get like this thread's concern. Anyway, the only electrical use I need in my coop is the water heater to prevent the water from freezing in the winter. Other than that, I don't have anything relying on electricity.
I would suggest that if anyone is depending on a thermostat to turn on/off a fan based on a setting, that they also invest in a separate alarm system for overheat situations. Nobody wants to hear that you lost all your chickens because the fan failed to turn on and the birds got cooked in the coop. That would be too much heartbreak for me. But I always design my systems to have a backup in case of primary system failure. If the fan fails to cool down the coop and the alarm sounds, then I would be the person going out late at night to open up all the doors and windows trying to save the birds.