Frozen pipes are usually the most significant issue with not setting the proper temperature in your vacant home. The water in your pipes freezes, water expands, and there's usually damage to pipes as a result. This can further lead to water damage throughout your home.
This type of damage is usually not covered by home insurance because it's considered your own fault. Your home insurance policy likely excludes frozen pipes.
Additionally, frozen pipes mean coming home to a lack of running water. Everything's frozen after all! Imagine coming from the airport to realize you can't shower, wash dishes, or do anything else that requires a faucet.
Cold and hot temperatures can also cause humidity, leading to mould and other issues. This usually happens when fluctuating temperatures create moisture. Cold nights that turn into sunny and hot days cause parts of your home to trap moisture.
Although unrelated to the temperature of your home, make sure to inform your insurance company when you vacate your home for extended periods. Home insurance contracts may be void when your property is left vacant. Vacant homes may require a separate, vacant home insurance policy.