When unions get to be that size is when they get captured by organized crime and other interests, because then they're acting as a de facto government and susceptible to the same corrupting influences but without the (by no means perfect but far better than nothing) safeguards we put on governments in modern democracies.

This is also why corporate monopolies are a fiasco and need to be prevented as well.

The general rule is "prevent any one group from consolidating too much power". If someone's solution is "let our group consolidate a huge amount of power" they're admitting they're the villains.