Uses expensive backup heat when the outside temperature is relatively high where the heat pump is much more efficient without it. You have to enter a false zip code in Florida, or similar (warmer temperature), for the thermostat to not improperly use backup heat. Nest "helps" people by unnecessarily driving up their electric bills, wasting energy. My $79 Honeywell WiFi thermostat in my vacation home is WAY better. It doesn't force expensive backup heat to come on when it's not needed.