The current leadership (Trump) is very racist and authoritarian, so I'm not sure that your confidence fits the current time;
For video, and I highly recommend this, try https://www.youtube.com/watch?v=Wf4cea5oObY
But if you don't like BuzzFeed or John Oliver, it's trivial to find articles about the harms caused by police unions from sources across the political spectrum.
In the abstract, conceptually, unions aren't a problem. I love unions! But police unions have such a long and consistent pattern of making police worse that I'm not sure any police union can maintain good things for more than a year or two.
I would be pro-police-union if police unions worked like all other unions, fighting for better pay and conditions for their members. That's just not how police unions work in the USA.
P.S. As you say, the article mentions a new police union that so far seems to be okay. It should be noted that union was formed after completely dissolving the existing police force of Camden, NJ, and all of its contracts, including the contract with the existing police union. Time will tell whether the new union starts to be a problem or not, but certainly starting under the threat of "act up and we will dissolve the entire police force again" is a good start.
Second, the union contract can effectively dictate how laws are enforced on the police, e.g., the what the disciplinary mechanism is like, so the laws become un-enforceable.
Third is through labor action such as work-to-rule.
I grew up in a suburb of Detroit at a time when the auto industry was contracting. What I recall was that senior assembly line workers (my neighbors) were working 60 hour weeks and pulling down overtime pay, while junior workers were getting laid off.
Naturally, unemployed people had no union representation at all, and this was a perfect opportunity for politicians to start stoking anti-union resentment amongst the working class, which persists today.
Meanwhile as I recall in one of the European countries, perhaps Germany, the unions negotiated with the government and agreed to a 4 day work week, in order to keep a larger number of workers employed. As reported in the US, this became fodder for "those lazy Europeans and their 4 day work weeks."
In the US, the car companies reacted by pulling the plug on entire factories and moving them to anti-union states.
I grew up in Flint, MI and my father was an electrical engineer at one of the 3 major automakers. We would frequently go to the auto plants for school field trips. I remember one particular trip where we were walking around the production floor and there was a guy just sitting in a chair sleeping right in the middle of everything. Nobody seemed to think it was a good idea to wake him up. Everyone just worked around him like he wasn't there. It was kind of surreal.
I'm not saying that proves the point in your quote above, but the unions were notorious for making it impossible to fire employees even if they showed up drunk or high or didn't do their jobs. They also didn't seem to be protecting workers interests very well, as their wages have stagnated for decades and they've lost the great benefits they used to have. It doesn't mean unions are a bad idea, but the auto unions seemed to be a particularly bad implementation.
In most cases, the net harm is actually minimal, e.g. increased levels of incompetence, drinking on the job doing paper work, forcing bosses to outsource, slower delivery times, etc.
In the case of police though, harm can be quite high when bad apples are protected.