In a sleeping bag which is rated for that temperature, sometimes cheaper bags are for like -5°C, which can be VERY uncomfortable already for +5°C.

(Just to make it clear I didn't mean to downplay this. Obviously to survive in your house at -10c you do need preparations which I understand many people don't have not being used to these temperatures. It's easy to forget as someone who lives a bit further up north that it's not a given that people have thick air tight walls and heavy down filled blankets to help them through. So to answer my own question: It would be unsafe because they might not have the things they need to be safe.)