If your fridge or furnace can be turned off completely by remote (or even locally), you're doing it wrong. Think for a moment what you are automating. The temperature, not the furnace. Your thermostat will be controllable, the furnace will remain just a dumb unconnected piece of equipment, but smart enough to remain operating within acceptable limits even if the thermostat is compromised. I have a fairly comprehensively automated home, but with full control or even the ability to operate devices outside their normal limits, you could do very little actual damage there, and cause a minor inconvenience at best. It's good to be careful and wary of any connected device, but at some point it's just fearmongering and/or a complete misunderstanding of the actual risks.
By the way, I'd be happy to accept liability for any damages such as the ones you describe, if I were selling you a home automation setup.