90% of NICS queries clear within minutes. The majority of the remaining clear within the delay period, a small percentage fail to clear during the delay period.
NICS is not intended to be a registry, it is intended to facilitate a background check in a timely manner. To the extent that the right data are loaded into the system, it does that job reasonably well. Where there are gaps in data feeds, for example from mental health authorities, those gaps are particularly problematic. Lots of examples of data connection deficiencies. Those need to get fixed. Retention time for the records is a different matter. Seems that the retention time should be sufficient to facilitate prosecution of offenses related to lying on the form. Denied transactions are retained by ATF for 10 years, and then transferred to FBI for longer-term retention. The statute of limitation is 5 years for lying on a federal form.
If you want a registry, that is a different discussion, but that is not necessarily an indictment of the NICS functionality.