The polygraph should be admissible because it is nearly fool proof and to get past this provide a control setting and multiple attempts at he same and varying questions. And to fool a polygraph is nearly impossible it either consists of shear luck or your Spock and cannot tell a lie.
I have repeated, first hand experience with lie-detector tests, and I would openly say they are in no way dependable. There are so many wild variables that may effect the outcome you'd have a better chance flipping a coin to decide if someone is telling the truth or not.
Maybe with vast improvements in their technology, a streamlined standard of approach, and a much more uniformed tester, they may become dependable. Even so, the person receiving the test is just as much a variable as anything else.
The polygraph is not almost foolproof. Many innocent people have failed them and many guilty people have passed them. Depending on how well you can control physical responses determines on how accurate the test may be, notice I say "may". It is not impossible to fool the polygraph, many people have done it by pressing their foot against something or their hand. This test has not been admissible to date because the machine's failure rate has been proven. Therefore it probably never will be used in court.
Any person who is nervous to take the polygraph for the first time will be nervous. And since polygraph tests records respiration, pulse etc. , a nervous person would already have an erratic pulse AND respiration. So might as well find him guilty...No need for a trial...Right? So basically a polygraph may not allow the defendant to be represented equally.