The FBI's ever-increasing collection of personal data -- which also includes a facial recognition system with nearly half a billion photos in it -- won't calm the fears of those worried about a biometric database being slowly built over time.
Obviously, privacy advocates take objection to the creation of any mass database with everyone's physical data for a number of reasons, not least of all are concerns about there being insufficient safeguards and transparency around which government agencies (and individuals) can access that data, and for what reason.
There's also potential concerns over racial bias among people arrested in the US. The prison population is roughly the same percentage black and white, which far from reflects the country's demographic makeup. Collecting iris scans could theoretically result in racial profiling, whether deliberate or not.
The FBI's existing database of fingerprints and photos has also recently been a bone of contention for privacy advocates after the bureau suggested it wanted the data to no be covered by the Privacy Act. Given that the San Bernardino "pilot" reportedly doesn't have any privacy impact assessment, either, there's little to ease the fears of those who are uncomfortable with how the government handles such data.