One of the reasons that unions in the US are unpopular because they function as the funding arm of one of the political parties. In some states you have to join these unions to get a job at a state agency or at a government funded school. This is obviously incredibly corrupting, where the unions donate huge amounts of money to politicians who then feed back more money to the union members, and it's all paid for by taxpayers.

There's also ridiculous rules that many of them have adopted. My favorite example is that in my state a teacher was awarded the "Teacher of the Year" award, which is a pretty prestigious award, and requires you to be nominated by your students. Before the next school year she was fired because they had to make some cuts and the union had set it up so it was a Last In-First Out arrangement. So the teacher of the year was let go as that was her first or second year teaching. The teachers union prioritized its older members at the cost of that school losing their best teacher.

American unions are very different, in my experience, from what I saw from the European companies I've worked for. Those employee unions seemed solely focused on the people working at the company and did a good job representing their interests and solely their interests. If that's how unions in the US functioned, many people would have a much different opinion of them.