[sokoloff]

The emergency brake in a car has a small fraction of the braking force of the service (normal) brakes. The emergency/parking brake typically acts only on the rear brakes and often via a separate braking surface that is nowhere near as powerful as the rear service brakes, to say nothing of the all-wheel service brakes.

[mikepurvis]

In at least some cases, it's actually the same pads and rotors but just cable actuated, so that it can still work in case of total hydraulic failure. Wikipedia:

"In vehicles with rear disc brakes, the parking brake either actuates the disc calipers (with much less force) or a small drum brake housed within the hub assembly (the inner circumference of the disc is often used instead of a separate drum). This secondary drum parking brake is often referred to as a banksia brake."

https://en.wikipedia.org/wiki/Parking_brake#Types_of_brakes