But if the person was trapped forever, and never returned to tell the tale, how do we know they went through at all, and didn't simply die someplace?
The classic tale of Rip Van Winkle, who went to sleep for 30 years, wasn't a story of fairy mischief at all, no matter how some have portrayed. They were granting his wish. Or should I say, Wish?
The character had a lunch packed and a good jug, and wandered off into the woods to escape his nagging wife. There he met with some of the Little People, and shared his food and drink with them, as a matter of good manners. They approved and asked, somewhat conversationally, what was his dearest wish.
He said he' wished he could just take a nap, and not have to go home to his good wife, who he loved dearly but who was always nagging him.
The tale includes one of my favorite quotes: "The human tongue is the only known tool which grows sharper with use."
The Good Folk granted his Wish, and he slept until his wife passed away.
But yes, there are hazards in entering the Fae realm. Be careful what gifts you accept, because you may be bound to reciprocate. Even the offer of a meal can leave one indebted, hence the warnings about eating or drinking anything in that realm.
By the way, technically "Fairy" isn't a being. It's a place, as in the realm of the Fae. The people are Fae, the place is Fairie or Fairy.
The last time the Bard entered that realm, he went voluntarily, but this was preceded by him doing one of them a favor, so he entered debt free, and left the same way.
And that should be the rule for any who venture there. Be prepared to pay your bills, repay any favors, and try to pay in advance for any bargains you make. Make no promises you can't keep.
A common trait of the Fae in fiction is that they don't lie. As in, they don't say things that aren't factual. That doesn't mean, however, that they tell the truth, or try to mislead.
As Jim Butcher described it in his Dresden Files books, "They don't lie, but what they can do to the truth is something else again."