He recommends a revolver for strictly personal protection because they work 100% of the time, and if something doesn't fire, just keep squeezing the trigger.
Everything mechanical is prone to failure. A better way to put this is a revolver is generally more reliable than an auto, but they DO NOT work 100% of the time. I've seen several revolvers in my lifetime that were locked up. unshootable. Try shooting full power .357 Mag through an airweight, and if you don't have a grip like a vice, by the third or fourth shot, there's a good chance the slugs have worked their way out of the round and have locked up the gun. I've personally seen this, and I know Massad Ayoob mentions it in at least one of his books. That's just one example.
Basically, I don't want you to get into the mindset that a revolver s 100% reliable. That will give you a false sense of security. I really can't believe a gunsmith told you that a revolver is 100% reliable. I'll give him the benefit of the doubt that perhaps you were paraphrasing him.