I would say they pass more counterfeits (30%?) than fail items that were witnessed by the submitter (20%?).
Those are my generalized numbers based on the message board posts I've read in the past 10 years.
There have been quite a few threads showing their ineptitude, including T206s being signed within the previous 2 years (because the same card was found in unsigned condition in a recent auction/sale online).
I've found multiple signed cards that were PSA certified that the name of the actress was spelled incorrectly.
https://www.blowoutforums.com/showthread.php?t=1407540
And then I've seen a reasonable number of posts angry that PSA did not pass autos they watched done by the player in person.