If anybody with enough motivation can modify any existing harmless image to have the same neural hash as a "tracked database" image this will create too many false positives. Too many false positives make the algorithm useless.

If someone with even more motivation and the means to put those images onto your device via social engineering, exploits or maybe even features and you become the target of a criminal investigation in any jurisdiction you just happen to be at the moment, this makes the algorithm harmful.