Correct. It's widely considered inadmissible as evidence in court. They're also rarely used outside of the U.S. AFAIK.

Recordings of interrogations that feature them also show that they tend to come along with other appeals to ethos ("this is doctor so-and-so...") and hand-wavey forms of intimidation like touting their "seriousness" or "efficacy".

Anything that requires a person to "interpret" its output is likely not very deterministic, and so likely not very reliable for establishing... anything. This is akin to palm reading or graphology.

Unfortunately, revealing to the police in such a situation exactly how foolish they look by getting behind such a device isn't exactly advisable either!