I think
@MudPuppy hit it on the head, dealing with corrupt systems sucks, no matter if it is corporate or a union.
I have heard similar complaints about what I'd call cliquishness. My uncle joined a union shop and was treated terribly because he wanted to work his way up fast. That was seen as disrupting the labor agreements. He stuck at it to get the pension (a union only benefit) that allowed him to finally retire after years of working hard manual labor, but he sure as heck isn't friends with any of his old coworkers. (As an outsider, my uncle is pretty affable and knows everyone in his small town, so I don't think it was him causing trouble, in this case.)
In general, when unions endorse candidates, the issues I think of they care about are A) government spending in their area of work (ie teacher want more education spending, construction workers want more public works spending) education funding (teacher's unions) and B) opposition to "right to work" laws (pretty much all unions.) It's a pretty straightforward, this is in our best interests endorsement.
Right to work basically cuts unions at the knees, because it forces unions to represent non-union employees without compensation. All unions do need money to hire the lawyers protecting you from unfair dismissal, which is ultimately why they collect dues (plus administrative costs, which is where corruption can seep in.) It also prevents employees being forced to join the union at whatever company. Which again, means the union collects less dues and has less money to represent employees against management, while simultaneously being required to represent the non-paying employees.
I'd be more in favor of Right to Work if the requirement to represent non-due paying employees was lifted. As it is, it encourages free riders who get the benefit of the union negotiating to better base rates (which the non-paying employees take advantage of) while contributing nothing back. I also think Right To Work proponents have a lot of strong arguments legally speaking, but ultimately I look at the unchecked power of corporations as requiring some employee lead bodies to check. Unions give employees power to whistleblow with more safety, to negotiate for fair wages, and to have a say in management decisions that effect them.
I don't have a union available to me, and it makes me feel less safe as an employee. It's one of my main drivers to FIRE, honestly.