It's definite that most police officers and most police departments operate in a competent and civilized manner. But human nature being what it is, there are bound to be some individuals who misuse authority. The case in Texas where a driver stopped for not wearing a seatbelt was handcuffed and dragged off to police lineup, is an example. If officers stop a car and then drag out its occupant and start beating him unnecessarily, the camera will record the whole incident. There are no real rules that say such a thing couldn't happen.
Some municipalities have cameras stationed on light poles in high-crime districts.
They claim a reduced the crime rate where the cameras exist, though some dispute the statistics. XXXV earlier this year, was recorded on police cameras. The police said they were looking for recognizable terrorists or prison escapees, but all they recognized were some known pickpockets. There was outcry when the news broke, but since the arena was a public place there wasn't much room to claim invasion of privacy. No one knows and there aren't many.