I don't think there is any particular answer to this question.
On the one hand, unions have long ago become business themselves. I worked in a union shop, long ago, while I believe the union helped at times, it was obvious that human nature and ego had it's effect on union officials.
Others here have mentioned the limitations unions infer upon the high achiever in the group, that is cause for some resentment.
Also, there are the folks who work jobs that will never see any benefit from unions. Small businessmen, and their employees are at the mercy of the economy and inflation. In their eyes the unions have gotten way out of hand, creating inflationary problems, rising costs, standards and the like.
I remember my father ( a small business man) watching union strikers on the tv in the 70's, guys standing in front of a house in the suburbs with 2 new cars in the drive complaining about their share, his opinion of that wasn't too high.
The bottom line is that unions ( not all, but many) did it to themselves. They got just as greedy as the employers they struggled against.
The original purpose was great, protecting the worker's interests, and this country surely needed that. But they didn't stop there, and soon created standards far beyond what the average non-union worker, with a decent employer, could ever hope to acheive.