I used to carry a very heavy chain in the my Blazer and then Bronco, and used it to 'unstick' people, and once to get myself unstuck. The trick to using a chain is to be smooth. With the chain I was using there was virtually no stored energy, because there was no measurable stretch. The main danger is in ripping parts off the car, in my experience.

Towing a car with a length of chain is pretty difficult. I have done it a few times, for distances of less than a mile. In all cases is was my 5000lb truck pulling a 3500+lb car. The last time I did this it was pulling an old (70's) Ford LTD. I let the owner of the dead car attach the chain on his end. Some slack developed in the chain, and when I rolled forward and took it up, the kinetic energy of the truck very nearly pulled the chain hook *through* his bumper.

As far as getting people unstuck, I would rather use a winch or a chain than a strap. IMHO if you need to use kinetic energy to unstick someone, you don't have an adequate solution, you have a dangerous situation. If I had a strap, I'd want one heavy enough that it wouldn't stretch much.