Comment by JasonFruit

In my experience, what they oppose isn't unions as such, but compulsory union membership. They consider that a union should arise from the expression of worker needs, and a union that they are forced to join and that exists whether they want it or not might lack incentives to represent their needs instead of the needs of the union. It's kind of like the saying about organizations in general: eventually they come to serve the needs of the organization instead of the purpose for which they were originally created.