The big unions are all corrupt as fuck and absurdly top-down and undemocratic. (edit: also a lot of people in the labor movement are grossly incompetent and gullible) This is because they've been shaped that way since FDR. The NLRA was designed to turn unions into state sanctioned and controlled institutions and then they were purged of anyone who had ideological commitments to democracy or socialism. From there they were allowed to collect dues from employees working under their contracts even if those employees were not members of the union which meant there was no material reason for them to actually do anything more than win an NLRB run election and a single shitty contract. Then they could embed themselves like ticks in job sites.

Today most unions are sitting on billions of dollars in pension and insurance funds. Union officials regularly make six figure salaries for nothing while their members make poverty wages. They are blatantly undemocratic institutions in which the members have so little say that the federal government consistently intervenes in elections. The unions do not have any obligation to recruit, educate, agitate, or organize their members. Most of the unions are only interested in getting Democrats elected primarily to fight right to work so that they can continue to extract dues from non-members. The unions also frequently suppress the self activity of their members and make it harder for them to organize and will even collaborate with management to remove problematic workers. When strikes or other actions do happen they happen at the whim of union leadership, not the members. They have been shaped this way by decades of opportunism and US government interference.

Unions do not have to be this way though. There's no actual obligation to go through the NLRA process. There's no obligation to work under the boots of the big unions. We should aim for strategic militancy not timid super-majorities. We should exercise working class power and democracy, not legalistic state power. We should fight like guerillas, not like liberals. Communists in the US should focus more on workers and sow our roots deeper into job sites, but among the 90% of American workers that are not organized in any sort of union who are generally the most proletarian workers, not among the 10% who are in unions. We should not seek to join bigger unions that will stamp us out. We and should replicate the successes of the CIO ninety years ago. If we are in the big unions already we should be organizing within them as communists, forming caucuses along our political orientations and tactically supporting reform efforts.