Small martial arts nunchaku are not all that dangerous unless you are skilled with them, and your opponent has nothing more than a knife. I hear that they are good against people with knives, though.
BTW, the typical demonstration moves (as seen done by Bruce Lee, Inspector Cleuseau and Cato, for example) are not really the way they are generally used for real. Typically they are held "chambered" or taut and ready to snap out, the advantage being that they then strike very quickly but the reach is not very long for that, so someone (competant) with a larger weapon like a club would be hard to attack that way.
I think they're the victim of over-broad Canadian laws designed to outlaw things like chains (or maybe just the legistators were really suspicious of anyone carrying a flail...), but it's pretty hard to make weapon laws that everyone can agree on. BTW the police museum in Vancouver BC (at the old morgue) has an interesting collection of confiscated weapons and confidence game equipment and other stuff, including several flails, submachineguns...
