There are no certifications for metrologists, so no easy way to determine the knowledge level of those performing the measurements. Since suppliers generally get paid the same for bad data as they do for good data (since the customer of the data usually can't usually tell bad from good) the data can generally be expected to be bad.
If the data is audited and compared to data for the same parts from a trusted source, then some correlation and possibly deficiency-directed supplier training could be done, but that all costs money, of course. This work should be done if the cost of dimensional problems will be high... A judgment call that all businesses need to make.
With the generally low level of GD&T knowledge and dimensional metrology knowledge, it seems that we're still in the "Wild West" stages of dimensional specification and measurement. It should get better over time as dense measurement data point clouds, improved GD&T standards, and very good software to evaluate the data become more common. Education and awareness in industry, and, I hope, awareness of the problems by mechanical engineering education departments, that may lead to ME graduates with better knowledge of how to specify a mechanical part's acceptable limits, should help too.
Quite a few people involved with this forum are working to make things better.
If suppliers knew that machine driven CMM measurement requires that they iterate as they establish a datum reference frame in order to gather repeatable data, that would reduce a significant part of the problem, I believe. The points they use to establish the datum reference frame need to be located relative to that datum reference frame in a repeatable manner and that cannot be done without iterating. The third iteration DRF is usually repeatable enough to be considered valid. The more imperfect the parts are, and the tighter the tolerances are, the more important it is to iterate when establishing the DRF.
Much of the training provided by measurement equipment representatives teaches methods for establishing datum reference frames, and methods for evaluating tolerances, that are incorrect. Their job is done when their customer can make the machine move and give numbers. The supplier's inspection department may consider their job done when the numbers they send don't end up raising red flags from their customers. It's up to the OEMs to first educate their own people, develop their own metrology capabilities, do correlations and probably then supplier training... That's all if they are aware enough, and care enough about their quality.
Thorny problem, I believe/agree.
Dean