This seems to presume that the members of this group have common interests to negotiate on (they dont) and to ignore group psychology.

If I have high individual negotiating power and I want to, say, work from home then I do not benefit from unionizing with a majority of people with low-power who do not want to work from home.

it's quite plausible that the minimum common interests the group has are already satisfied by the employment terms, and then, the negotiation is an opportunity for "union reps" to game the system to the advantage of some over others.

You iterate this over a large number of negotiations, thrown in "union" as now a group and not a mere negotiating block, and you get the police union.

The first sentence is true. The second sentence can be true but often isn’t. If 90% of the workers at a firm are worth $50,000 a year and 10% are worth $200,000 a year a union will represent the average worker, screwing the top 10%. The closer you are to the average of the members in your union the better they represent your interests. If you are far from a representative member the degree to which they will fight for your interests is markedly less.