I tried using the the handbreak from about 20 km/h in an empty parking lot (if you expect to use it in an emergency, you should try it first, shouldn´t you). It was a painfully long distance before the car stopped. Instead of the almost immediate stop with regular breaking it took about 15 m. The car was an old VW beetle. As the CG is pretty far aft, it stayed pretty stable. When I tried this with a more modern car, the rear brakes had the tendency to lock. That made the distance for stopping even longer. Most modern car have their CG to the front. That way the tend to understeer.
If you count on using that technique, you may wish to try it when you still have alternatives.