And much good it did for preventing the Snowden leak. The false sense of security instilled by confidence like yours likely hindered implementation of common-sense safeguards, such as the two man rule and more generally any functional oversight of any kind.
In my experience, the NSA performs its own security clearance checking and does not use the OPM for this function, but, in any case where OPM did perform a background check, that background check provided China with far more compromising information than China otherwise would have had.
That people place such faith in vetting in the first place is a huge problem. Furthermore, background checking performed by most of the rest of the US government has actually been a huge source of actionable intelligence for our enemies.
In my opinion, it is not possible for an organization the size of the NSA to keep its secrets. It's, what, 1000x larger than the Manhattan project? And, would you say that James Clapper's intellect was in the same ballpark as those of Leslie Groves and Robert Oppenheimer?
In fact, when confronted with opportunities to avoid mistakes, the US government's reflexive response is compulsion toward secrecy so as to avoid reform. IE, so much of this secrecy is motivated by desire to escape oversight, rather than to provide our nation actual advantage over rivals. If the latter were our goal, oversight and strictly limiting classification to the most sensitive subset of weapons research and ongoing military operations would be a road to it. Instead, such approaches are termed dangerous and even intellectual property normalization treaty negotiations are treated as military secrets in order to protect the power of entrenched political influences.
When secrecy is so rampant that it makes it impossible for us to keep secrets, you might ask why we have so much fucking secrecy.