True, any lock can be broken. The trick is to put the barrier up so high it simply isn't worthwhile. I have a couple of very expensive bikes. I keep them locked up in a shed and secured to the concrete floor with a 16mm Pragmasis chain, guaranteed unbreakable with 42" bolt cutters. Sure, someone can come in with an angle grinder, bringing their own power source, use up a few blades and risk serious injury cutting it without having the chain secured in a vice - but is that really a viable option? Me thinks not. Me is betting not, in fact.
These chains are very expensive, but so are my bikes. And it isn't just monetary value - I really, realy, really do not want MY bicycke stolen.