At the end of the day, as it is here, the designs and materials that we try to evaluate are only vaguely suggestive of a frame or component's reliability and flexural performance.

It really does come down to product testing under varied conditions and extended intervals, as well as the quality control inspection/testing that follows.
It is in the end a matter of the manufacturer's reputation that counts, above all else, though I myself remember flex-testing a PX10's handlebar and rejecting it's stem's worthiness from the get-go.