It’s called a parking brake rather than an emergency brake because it is not designed to stop the car, only hold it in place once it’s at rest. As a matter of fact, most modern cars with rear disc brakes use a tiny drum cast into the center of the disc and tiny shoes for their parking brakes. It does not have sufficient power to stop the car in any meaningful distance by itself.
Having said that, should I ever lose my hydraulics entirely I’M PULLING THE HANDLE! My parking brake WILL be used as an emergency brake even if it fries trying!