For cold temperatures, heat pumps struggle to keep up because it is harder to extract heat from very cold outside air than from warmer outside air.  

We don't winterize, that's when I do any maintenance it needs or we spend a night etc....I have cameras, temperature notifications, leak detectors that will automatically shut the main off etc....and the mini splits are back up heat.  

The temperature is 80 degrees in their house? 

I can control the temperature of the house, the locks on the doors, cameras to see what's going on outside, sensors for moisture/freezing in the basement, door sensors for doors that shouldn't be open and a lot more.

In general, most materials degrade more quickly with more extreme temperatures (either hot or cold), and they degrade more quickly when humidity is high or when there is a lot of water in the environment...by not having power and AC/ air circulation, you're subjecting the house to more extreme temperature swings, and more humidity; both of which can cause problems.Also, as others mentioned, if the house appears vacant with no lights on, it will become a target for burglars, vandals, squatters, etc.Was the refrigerator / freezer completely emptied and cleaned before power was turned off?

People are keeping temperature set around 70F in most cases.

Temperature gun reading all over the place, inside and out. 

I would still make sure you get the cold temperature rated version that don't have to back up heat generation in freezing temps. 

We have a Airbnb outside of Detroit, what is your opinion of what temperature the house should be set at before the renters get there?

Never had an issue but am told (the house is professionally managed) that maintenance on it is intensive between checking chemicals in the water, maintaining the water levels with heavy use (and water temperature by extension since it takes 24 hours to heat up to normal operating temp after refilling), filters, hair, other cleaning etc etc etc