While he was at the NSA, Edward Snowden complained that it stores more information on Americans than on Russians. He complained that that's illegal, but there's another remarkable facet to it: How good is the NSA at collecting information from Russia, then? I can hardly believe that the NSA tries to collect more data on Americans than on its actual mission, so how good is the NSA at its mission?
There are many other examples, like the German service that's supposed to monitor the nazis and missed the a group that made and sold a DVD about its killings.
It seems so strange to assume higher-than-average competence in organisations like that.