Unions, as a concept, make sense, but the big issue that you run into is that for some reason our society recognizes that a single organization controlling the supply of, say, steel is bad, we don't extend that recognition to Labor.
You see, the issue with Unions isn't Unions, if Unions were restricted to being strictly for a single company's employee's they likely wouldn't be nearly so corrupt or bad (nor anywhere near as large), but we've allowed certain unions to become monopolistic of labor in specific industries. Take the UAW for instance, they control the labor markets for automotive workers throughout the country save in states with right to work laws. This scale makes it so that they do not have to truly be concerned with the health of the individual companies, if one car manufacturer goes out of business, the UAW doesn't care (even though individual workers do), as it just means abit less money coming in for them. Thus, when those automakers run into issues, the UAW doesn't really have any reason to negotiate with them, in fact, driving them to death actually serves their interests more since they can hold up a company dying as a point in their favor, so they screw over part of their membership just to leverage more power for others.
And that private sector unions. Public sector unions are even more parasitic and, I'd argue, outright evil...