How safe are corsets? the real one's especially the steel bone one?

Probably: Corsets are body modification garments which do put pressure on the parts of the body over which they are laced. You cannot realistically lace it to tight enough to cause internal damage but you could restrict breathing enough that you might not be getting enough air. The tightness depends upon good communication between you and your partner.

Depends: Why are you considering wearing one? If it is for fashion and it fits your body it could be safe . Make sure you can breath. Unless you have a serious medical condition like internal injuries that need to be immobilized you probably do not need it. The steel bone corsets look a little uncomfortable and I am sure it takes some time getting into and out of...

They can endanger : Your health. Tight lacing (also called waist training or corset training) can cause shortness of breath, fainting, indigestion. This practice can decrease lung volume (requiring intercostal breathing). Relatively less use of lower lungs can leading to coughing. Some corset styles can push the lower ribs inward. The liver is forced upward ; accessory lobes of the liver may form. Stomach volume is.