I think where it breaks down though is if a company manages to monopolize the market we all recognize this is bad. If a union tries to monopolize the labor supply to a company, most pro-union opinions celebrate this and argue the company should have to negotiate with the union to find a rate rather than being able to just shit-can everyone in the union and move on to the next guy.

Union itself I'd agree could function as basically a corporation of workers. That's not on face a bad thing, but the devil is in the details of what kind of violence (via law or otherwise) is used to try and use that to form a monopoly. Of course the companies are no better in this regard, they use the violence of the state to monopolize markets as well.