I was a bit surprised to
find that my home thermostat was running BusyBox.
My thermostat contains about 2 dozen parts, even if
you count every nut , bolt, and washer. It does the job
and is not hard to understand or repair if/when it needs
it.
Quite why I would want a thermostat with presumably
several million components, running a multi-user operating
system is, to be honest, beyond me.
-tony