The reasons that people believe these items to protect against spirits/evil and fae (respectively) is because salt is believed to be pure, therefore a circle of "purity" couldn't be crossed by evil. And iron is believed to repel fae because (I'm assuming, I haven't actually done research on it) is because iron is what really gave humans an advantage over our world. Technology, weapons, blah blah blah. Fae are spirits of nature, and nature is slow to change most times, iron was a catalyst for change so people assumed that nature (and therefore the fae) didn't like it. I think the easiest way is to explain it is that Iron and Fae have opposing natures.
I think they're good protection. But they wouldn't repel "everything."