Yes, many people have died unintentionally from CO due to faulty furnaces while they slept. If you're intentionally creating copious amounts of CO by burning charcoal, and purposefully capturing that CO in a small space, as long as you're creating enough CO, a person should fall unconscious quickly and not even be aware when death occurs. But, a person needs to read as much as possible about the method and take care to do it correctly. Tents need to be prepared to some degree in order to keep contained as much CO as possible, and enough charcoal needs to be used. These are things that are learned by researching the method. The biggest "con" about the method is that if it isn't done correctly, meaning someone has an unsuccessful ctb, then the person could end up severely neurologically damaged (a vegetable) in a worst case scenario. Bottom line it needs to be done right and not screwed up. Because of that, it's not for everyone. But, it's pretty close to ideal if it is done correctly.