I'd say Occam's razor is useful in most cases. It runs into problems when you're dealing with a human element, because humans do things that are messy, irrational, or convoluted.

If your car won't start on a winter day it's probably safe for your first assumption to be the battery isn't putting out enough cold cranking amps.

If somebody gets busted for shoplifting there's a whole slew of reasons they could be doing it, some of them completely insane to an outsider, and most of the explanations are roughly as likely as any of the others.