Anything that drops your blood pressure very low or causes shock risks brain damage, including blood loss.. Think of like this. Why do people get resuscitated? If someone has an advanced disease, virus, cancer, or massive trauma to the organs, restarting their heart alone is not gonna do much. You have to treat whatever is causing the heart to stop eg. the poison. They could try but if the patient revives - the original cause of death hasn't gone anywhere!
In case of toxicity, if the patient is at the stage of cardiac arrest the poisoning must be advanced and successfully treating it is unlikely. So at the stage of serious brain damage (no oxygen = brain cells go RIP) chances of saving that person to end up with someone live but brain damaged is low.
OTOH for hanging, drowning CO the cause of cardiac arrest is usually very obvious and simple to remove eg a ligature around the neck which anyone can cut off, and once you do that oxygen resumes flowing and chances of successful resuscitation are relatively high. So EVERYTHING that can kill you will cause some kind of brain damage, the risk depends on how easy it is to revive someone after they've reached the point where the brain cells are beung deprived of oxygen. And with hanging, you are obviously starting with that bit, so risks are higher than for other methods.
