I don't know, even if the technology would be that far this thing would still worry me quite a bit. I would rather much prefer if they adoperate this robots in other jobs, like in mines or factories.
Another ethical problem with having them as policemen would be for privacy, they would be able to track the movements of every single citizien, we already live in an Orwellian society, with cameras everywhere, I'd rather much prefer if this situation doesn't degenerate even further, but that seems inevitable nowdays