The question is correlation between checkride failures and negative overall performance. For some there's no correlation. Akin to the SAT, GRE, LSAT, etc. Not everyone who flubs a standardized test will flunk out of grad school, etc. Some don't test well. Yet, some will be denied admission because of standardized test performance. It does not take into account motivation, willingness to improve, luck, etc.

I spent 4,000 hours as a CFI in part 61 schools and was floored by those that failed a ride vs. those that passed. In some cases I would put money on a pass. Result...fail. Others that technically met the PTS but barely...pass.
Before long I could give you the examiner's oral question verbatim. Trained two pilots that are now at majors that I consider excellent pilots that failed checkrides. One Chandelled through a cloud on a commercial (a dubious decision given the ceiling) the other got confused on an NDB hold (the old school stuff - no RMI. Remember the "when the intercept angle equals the deflection"?)

Point is whether checkrides are a predictor of performance. For some it is. Others, a lousy bad break.