Well, they can be effective as a prop to psychologically manipulate the subject into thinking you know when they’re lying, to try and pull out a confession. But that mostly only works if the subject doesn’t know it’s pseudoscience and believes polygraphs actually work…
From what the anti-polygraph people say, apparently it only takes about 30 minutes of training before most people can actually manipulate the polygraph in a controlled way.
Which means every cop will know how to get past them. I grew up in a family with a number of members in law enforcement and other parts of the justice system. Cops actively teach each other what to say/what to do in court and when they themselves are questioned.
The county I had lived in at that time was the worst. The guy in the sheriff's office that administered the lie detectors was "criminal scum" himself. For the longest time there were allegations of criminal behavior, but he finally pissed someone off and they hired a PI to follow him around. Caught this 40+ year old member of law enforcement giving minors alcohol at a party and hitting on them. It broke on the news and he was fired from his law enforcement position, but no criminal charges where ever brought...
What was even better, he was quietly brought back on as a consultant to administer lie detector tests!
From what the anti-polygraph people say, apparently it only takes about 30 minutes of training before most people can actually manipulate the polygraph in a controlled way.