Accuracy of measuring what? Despite the term, they don't actually detect lies. At best, some of them can detect physiological "events", as when a particular question or word induces acute anxiety. Most interpretation of those events is highly subjective.

The closest thing I've heard of to an objective method, the guilty knowledge test, doesn't center on evaluating the truth of the suspect's statements at all. Basically, the authorities know certain confidential facts about the case, such as the location of a wound, characteristics of a weapon, characteristics of an item stolen by the perpetrator, non-public "calling card" of a serial killer, method used to break into a building, etc.. The interviewer (who should be unaware of the true fact, to prevent something like the Clever Hans effect) is given a list including the true fact and plausible alternatives to incorporate into the questions. The idea is that a reaction specifically to the true fact indicates that the suspect knows something that only the actual perpetrator would know. As far as I know, interpretation of these results is still ultimately subjective because there is simply too much variability in human psychology and physiology to define a useful objective standard of when the test is "failed".

Terrible (50%?). They're so bad their results haven't been admissible in court for years.

I don't think they've been shown to be better than placebo.