I've played with my driveline parking brake while going down hills before. It helps, but it doesn't provide significant stopping power while cruising. It is, after all, one small drum brake. I'd say it's roughly the size of a brake drum you'd see on a 1/2 ton pickup. And of course, there's only one of them vs four on the wheel. AND the drive shaft is spinning much faster than the wheels. Sure, use it in an emergency, but don't expect a whole lot
Note that any modern hydraulic brake system is split into two branches feeding two wheels each. On my International the split is both front to one branch and both rear to another. If a line snapped at the front wheel I'd still have the rear brakes. Unless something catastrophic happens to the master brake cylinder you will have some braking capability.
Also, mine has an electric brake booster so that if the engine dies you still have brake boost.