The main problem is that if the outside temperature goes below a certain level, the fridge thermostat believes it is cold enough and can turn the compressor off, causing the freezer contents to start to thaw. The contents of your freezer may thaw out and then re-freeze when the outside temperature warms up without you even knowing.
For a garage or outbuilding location, you really need to think about getting a freezer that will work when the ambient temperature goes as low as -15C or 5F.