Dangers of the Emergency Brake
Even though the emergency brake cable is housed in a protective sleeve, with infrequent use, the cable can become corroded and rusted. This can result in cable failure just when you need it the most. Normal use prevents buildup and keeps the cable in good condition, though you should have the emergency brakes routinely inspected, as they sometimes need to be tightened [source: wiseGEEK].
In cold temperatures, the emergency brake cable can become frozen and fail to release when the lever is disengaged. Parking in a garage or other protected area can help you to avoid this problem. But if you have to park outside and it freezes up, you should not attempt to drive your vehicle. Solutions include waiting it out or jacking up the car and using a hairdryer. The best option in freezing weather is to simply not use the emergency brake at all [source: USACE]. If you're on level ground in a manual vehicle, put the car in first or reverse and skip the e-brake.
Be aware that in some vehicles, the emergency brake engages the front brakes, not the rear brakes. Knowing which brakes are set and properly chocking your vehicle wheels will protect you in situations where you must jack up the vehicle. Refer to your service manual before you raise the car. Believing the rear brakes are engaged when they aren't can be extremely dangerous.
Now onto the big question: Is using the emergency brake safe when the other brakes fail? Yes and no. Pulling the brake lever quickly will cause the vehicle to fishtail, lock up or skid, essentially removing control of the vehicle from the driver. But if you're ever in the highly unlikely but extremely serious situation of having your service brakes fail to function, try to stay calm and pull the emergency brake lever up slow and steady, bringing the vehicle to a longer but more controlled stop.