The only way polygraphs work is by convincing people its an actual lie detection machine. Cops leveredge this belief and tell you that you "failed miserably" so you ultimately confess because "your caught". They are about as accurate as flipping a coin.
This is not accurate. Polygraphs are not detecting lies, they're used to assess your sensitivities; there are really talented interrogators in counterintelligence, whose full-time job is to fuck with you in subtle ways. To poly a person at will is very much a power move, and some guys fucking love it. But that's a different story all together. Most of the time it's a formality like everything else. In reality, people…
Useful in what sense? That you can't figure out anyway what tested person is capable of because tested person can believe that they have skills on godlike senior level, but they are junior at best?