Dirty little secret, according to my daughter who was high up in law enforcement for several years, defendants who pass polygraphs are seldom prosecuted.
And the vision/visual deciphering which is fairly new is good tool as well.Also a strong indication of guilt or not.
A good friend of mine administered polygraphs for years. He admitted that without a really good examiner it "can" be beat but with one it's a lot tougher.
If Cain wants to survive, truth is his only friend.
I don't expect the people that use them to claim otherwise.