Unions are prone to corruption and abuse just as any other human institution is. Maybe it’s a Churchillian, the-worst-there-is-except-for-all-the-alternatives kind of situation.
What I think is clear is that (1) unions are largely responsible for many of the improvements in the lives of workers during the 19th and 20th centuries that we now take for granted and were instrumental in the creation of the social safety net, (2) there was significantly more economic equality during the heyday of union influence (I think that’s a good thing), and (2) without unions, there is no plausible candidate (at least that I’m aware of) for providing an institutional counterweight to the influence of the rich in our politics.
Or, in the words of Kevin Drum:
Unions have lots of pathologies: they can get entranced by implementing insane work rules, they can get co-opted by other political actors, and they can end up fighting progress on social issues, just to name a few. But they fight for economic egalitarianism, and they're the only institution in history that's ever done that successfully on a sustained basis. That's what makes them so indispensable to liberalism and that's what makes them the sworn enemies of conservatism.
Or, in the words of Joe Hill, as sung by Billy Bragg:
Lest I be accused of a naive view of unions perhaps it would be useful to consider them in light of the reading I've been doing about "the powers". What follows is only a rough idea. Suggestions for refinement are welcome.
The powers are those institutions or structures intended to serve humanity but, in their desire to take the throne of God, turn against humanity and seek to enslave them. The powers become demons whose sole desire is survival.
Unions are one of the powers. Their desire to survive is the source of those pathologies mentioned by Drum. As one of the powers, they are in rebellion against God in this era of the Fall. It is for this reason that we should not put our faith in them, i.e., we must not become idolaters. If we do we will become as enslaved to the power of death inherent in them as any fanatic is enslaved to his ideology or bigot to his race.
Though I can't speak for Yoder or Stringfellow or Wink, I do not believe that this theology of the powers necessarily means we must (or even can) separate ourselves from all powers for fear of their nature. Our task is to live humanly in the midst of the Fall. This means that in the course of our fight against dehumanization we will sometimes find ourselves on the side of one or another power. As long as we do not become their servants I see no problem with this.
So, yes, unions are not innocent and will do whatever is necessary to ensure their survival - exactly like any other power. But corporations and right-wing ideologies are powers as well. Unions have been and may yet again be a source of resistance to these forces of dehumanization.