Disclaimer:
I've made a career side stepping unions in various industries (public unions for hospitals, transportation/trucking unions, medium-sized online retailers on the cusp of unionization).
Unions have given corporate a new legitimacy.
The 'human abuse' component in unions is quite substantial. Internal groups tend to formulate inside unions capable of influencing decisions, leaving a minority without real representation (primarily new union members who don't get grandfather clauses, this minority becomes the majority with no power due to legacy senior union members; seniority is measured by hours worked, not expertise or merit keep this in mind).
Many times this abuse is for personal gain like getting the easier jobs or influencing the hiring process to get their friends/family jobs for example.
I make it my job to get chummy with senior union reps. By using my leverage to help a union rep get his cousin hired (through influencing HR or interviewing managers), I have shielded myself/the company from potential genuine grievances (forging allies). The cousin/whatever is probably aware of the game as well and knows not to rock the boat too, double-plus-good. This is more common than most would want to admit.
Another neat trick to controlling unions: once enough of the legacy employees, who can sway the unions decisions with their SENIORITY (the golden word), fall to your side all one has to do is hire lots of people who don't speak English. The legalese in most union handbooks is worded for lawyers really.
Older union guys don't care about the new blood to inform them what is and isn't okay. Being human they also appreciate that someone else is doing the hard jobs they don't have to.
Ever wonder why unions don't fight to make union handbooks easy to read? Because that would REALLY hurt corporate. Doctors and engineers are forbidden to unionize because they can read. Straight up.
Regardless unions are damaging to labour forces. They reduce efficiency, competence, shatter budgets, destroy the ability to become an entrepreneur (because money isn't tight enough gotta also pay union dues on top right?) and ruin any form of scheduling/time management arrangements. Once a bad culture sets in it is near impossible to root out (its easy for bad culture to set in when alcoholics can't be fired and must be given multiple chances; morale drops for the rest of the employees when they witness such things).
It's one thing to petition for humane working conditions, it's another to get paid for not working and boy did I see lots of the latter.
Don't listen to me, take a part-time summer (or night) job for the public sector with a union. Witness the absurdity yourself.