One of the main concerns is mistaken identity and just the idea that an innocent person could be apprehended, accused and even sentenced for a crime they didn't commit. That's clearly an error that we want to avoid, and we also want to avoid the opposite error of failing to apprehend someone who could be a great danger to other people.
That's not new. We've been trying to mitigate those problems ever since we've had eyewitness testimony, but it takes on a new form at the scale that face recognition technologies are being deployed. To my mind, that's the main difference.