You raise an interesting conundrum, but you can tell of a certain success rate by the number of liars who eventually confess to lying or are found to have been lying.
Wouldn't that be just the same case as with the caught liars but extended to a group of people who despite being successful at the interview failed fulfilling the task? There is still an unknown group of "liars" who passed the test and never confessed.
We're talking about legal contexts here. Almost no one, unless coerced/brutalized by police/DAs, admits to crimes they did not commit. And the you have the thing where you can verify their admission of guilt with corroborating details.
There have many many many cases of individuals getting behind bars because they have been coerced to plead guilty and confess to crimes that they never committed.
"unless coerced/brutalized by police/DAs" is not really exceptional, once in a decade, type case, sadly it is rather routine.
I know personally a person who plea-bargained, which means admitting to a crime they didn't commit, because the likelihood of being convicted (and the penalty of conviction) was so great a threat. He pled to a lesser crime (that he also didn't commit) and served 5 years.
Sadly, there is a systematic problem in which many must plead guilty because they will be punished beyond measure if they do not. It is structural and endemic in American justice:
