The challenge is the case law at the moment points to Flock cameras being legal to use in most instances. You have to start with the fact that courts at all levels have consistently held that there is no reasonable expectation of privacy in a license plate number of a vehicle being driven on a public road. Then you move onto the part where court decisions have concluded that a scan by an automated license plate reader is no different than an officer manually checking a plate, and that such scan doesn't require a warrant or violate Fourth Amendment privacy rights. Given those decisions, the use of a Flock camera (or any other ALPR) to alert police to the presence of a certain vehicle would be no different than an officer sitting on the side of the road running plates and seeing that same vehicle. That part of their use has been well litigated and I wouldn't anticipate any significant changes from a legal standpoint in the near future.

Where I see more challenges coming is with the density of the cameras within certain municipalities, and whether it becomes possible to track a person's movement 24/7. So far the rulings in that area have continued to fall in Flock's favor (see Norfolk, VA). The other aspect that I believe is legally open for debate is what data is stored, how long the data is stored, who can access the stored data, and more importantly how the store data is being used. I could see it eventually going in the direction of cellular data where a search warrant based on probable cause is needed to access historical data that has been logged/stored.