They're a reaction to when the balance of power tilts too far in favor of employers, an imbalance that can get very large depending on the economy and the company.

A world with no government labor regulation and no workforce organization would rely almost entirely on market forces and the goodwill of business owners for workplace conditions. Picture that (and if you say, "Well, people just wouldn't work at jobs with poor conditions," I will laugh).

The issues arise when the power imbalance tilts too far in the other direction, and you get the kind of abuses and frustrations that random thoughts mentioned.

However, saying "Sometimes unions have corruption" is not a good reason for abolishing unions, as "Sometimes corporations care less about the welfare of their employees than profits" is not a good reason for revolting into Communism.