They are equivocally, with no shred of a doubt

#not

accurate.

Not only that, they can easily be fooled. Like, stupidly easily.

They need to measure a "baseline" to get your "at rest" heartrate, breathing, body temperature, etc. Just hold your breath for a bit, this can fool the baseline.

Fun fact:

William Moulton Marston invented the polygraph test.

He and his wife created Wonder Woman.