Unions are constantly doing evil things, usually for the same greedy and power-hungry reasons as any corporate board, and to the same detriment of society at large, but because we think of unions as being on "our side" and latch onto this romanticized notion of the unions of the 1800s they get a pass.
The police and prison guard unions constantly do horrible things to increase incarceration rates, erode civil rights and remove accountability. Labor unions fight against improvements that would make their industries pollute less if it makes their jobs harder or would replace union-dues-paying members with technology or non-union jobs. And they constantly create artificial monopolies and cartels far worse than any of the corporate behavior, which create situations like the pilot unions where senior pilots get huge salaries and cushy routes while entry-level pilots are treated like slaves, or where you're not allowed to plug in your laptop charger to a wall outlet because only a union electrician can touch it.
I wish society was better at recognizing "this group is harming society for the benefit of themselves" can be applied everywhere- corporations, unions, branches of government, etc