Unions used to have vibrant, militant leftist factions that brought the vigor, courage, and zeal to fight like hell for the best working conditions and wages; it were these radical militants utilizing direct action that helped secure the economic prosperity of the post-WW2 Boomer era. However, the red scare and anti-communist witch hunts of the 50's drove out most members of these factions from unions, which resulted in many unions, run by moderates, to become weak and complacent, prone to fighting with each other over jurisdictions and neglecting the workers who voted for them. We saw this in the John Deere strike, we see this in the unionized grocery stores with the shit pay and constant struggling of grocery workers throughout the US, we see it in the decline of unions in general and the cowardice/resistance one usually faces in the labor movement when they suggest labor actions that aren't fully protected under the law.
Moderate liberal/centrist factions, who lack the solidarity, culture, and political analysis needed to foster a vibrant, united working class movement has continuously shown to be a detriment to the labor movement. While expanding our reach (breadth) through unionization is necessary, we also need to take control and bolster the movement from within (depth).