Rule #1: Unions are working as a countervailing power against a much. much more powerful entity. If you are criticizing them in such a way that you lose sight of this, you are spreading anti-union propaganda.
It's not that there aren't or can't be problems, not that there can't be criticism, it is that you are criticizing a _much smaller and weaker entity_ whose purpose and existence are rooted in fighting the much bigger and more powerful entity.
That is the lens.