Are we good at epistemic but not instrumental rationality?

Holden implies (and I agree with him) that there's very little evidence at the moment to suggest that SI is good at instrumental rationality. As for epistemic rationality, how would we know ? Is there some objective way to measure it ? I personally happen to believe that if a person seems to take it as a given that he's great at epistemic rationality, this fact should count as evidence (however circumstantial) against him being great at epistemic rationality... but that's just me.