Going off the "how/why?" stuff from last night, I'd say there are two big reasons contributing to cops like this dude being able to coast through complaints with no incident for a long time.
1. As said before, the police unions tend to be a big problem because they take the "represent our people" too far, where the only thing that matters is protecting them -- essentially turning the profession into a mafia-style thing where, once you're part of the "family", you are gold. Look at the aftereffects of the Eric Garner killing. The minute action was taken against the cop who killed him, the union heads went full retard, acting like they're now facing dire consequences for just doing their job like the dude was in trouble for no reason at all.
2. The overabundance of love/respect for the badge. Whenever some incident happens, you can count on a lot of people taking the "I lean towards always giving them the benefit of the doubt" more often than they would if a member of a more civilian profession fucked up at his job. Always found that weird. They're human and, therefore, they're capable of mistakes AND they're in an often-stressful job, which can easily lead to mistakes and cause those mistakes to snowball into big incidents. So, placing them on some sort of pedestal where they automatically deserve the benefit of the doubt in what looks to be an unnecessarily violent situation seems to be choosing to be willfully blind.
So, you have a profession with a strong union that will go all-out to protect them and a populace who will gravitate towards taking their side unless what they did was so over-the-top there's no logical way to justify it. I'd think that would lead to problems with most people, being in a position where you have power and public opinion on your side the vast majority of the time -- makes it pretty easy to slide into doing what you want instead of what you should do.