Polygraph machines are about 70% accurate, even those who administer such tests don't claim beyond 90%.
A few decades ago they were legal to use in all workplaces in my state, I took a job in a place where the boss forced everyone to take one. I messed with it on a couple of questions just to see if it worked. I created a false positive and lied about at least one thing as well. Other people were accused of lying when they bared their souls. Still others lied on everything, including one guy who'd been convicted of theft and got away with it. Not only was it a joke, it was a absurd expense as this guy ran through so many employees that by the end of the year when I quit, nobody who worked there had even met anyone who worked there when I started.
The only thing I've seen worth even less are those tests which as questions like "what percentage of cops do you think are crooked?" which weeded out every single guy who was smart enough to second guess himself while answering. Every ignorant moron passed it, every trained, competent guy failed.