Large Scale Use
Whilst it can be agreed the impact of using FRT at large gatherings is non-intrusive, maintaining a database of attendees is starting to get in to the same territory as the Fed. Government demanding information on anti-trump websites and visitors. Scarily, this could also be used against people to silence any political critics.
I was at a music festival a few years ago (Download festival, UK) and they actually went out of their way to advise people FRT was in use, and images would be stored for trial purposes (they didn't have a scope or documentation showing the extents of the permission gained, so one can only assume the images are still being held/used). Despite being a law abiding citizen, this disturbed me. I'm not convinced police can get a warrant to monitor every person in a city, so how can a warrant be provided giving the police blanket rights to record "just in case" something happens?
If this was for public safety, the FRT would be on the gates, prior to allowing entrance, would search for previous offenders - with non-risk images being removed. This is nothing but an escalation in the monitoring state. Welcome to 1982, only a couple more years to go...