There are many grades of chain, some are very hard. There are many types and sizes of bolt cutters some compound types are extremely powerful. And there are small battery powered carbide disk cutters that will cut through almost anything.
This type of lock attached to the big chainring trapping the chain will dissuade most thieves, and are immune to all but the biggest bolt cutters.
[image]http://www.cwylde.co.uk/bikes-motorcycles-shop/motorbike-stores-locks-solido-100d.jpg[/image]