Alternately, in such a world, it could be that improving your predictive capacity necessarily decreases your ability to achieve your goals.

Hence the classical example of Cassandra, who was given the power of foretelling the future, but with the curse that nobody would ever believe her. To paraphrase Aladdin's genie: "Phenomenal cosmic predictive capacity ... itty bitty evidential status."

Yes, a Zelazny or Smullyan character could find ways to subvert the curse, depending on just how literal-minded Apollo's "install prophecy" code was. If Cassandra took a lesson in lying from Epimenides, she mightn't have had any problems.