There may be some truth to it. A person gets stuck in NC when his/her NC search generates a hit in reference files. Reference files do not necesserely list bad people, but contain all records relevant to FBI activies. E.g. if you were a witness of crime, or was a crime victim yourself, then your name may end up in FBI reference files. So the longer you stay in the US, the more likely a possibility of your name making its way to FBI ref. files for some innocent reasons. Then, once your NC gets a hit, it's put aside for a manual retrival of a relevant reference file, and this is when NC black hole hell starts: too few unmotivated clerks are manually processing too many NC hits.
There is also another component to the NC backlog, and this is USCIS itself. Once FBI retrives a refence file with a hit, it sends results to USCIS (to fraud detection unit in DC), it's done with your namecheck as far as they are concerned, and now it's up to USCIS to decide what to do with the information they got from the FBI. So that's why you see people reporting their NC cleared by the FBI, but USICS still tells them that background checks are pending.