The two big arguments I see against unions are
1. lazy workers
2. corruption
And in the case of 1., I mean come on. Lazy workers are everywhere union or no. In the case of 2., yes this happens. Wage theft from the worker by management happens probably a lot more, on the other hand. It impacts at least 1/3 of minimum wage earners in cities like LA and Chicago, and for those who have experienced a pay violation on average they loose out on 12.5% of their actual paycheck (1). Just look at the second page of this report and see the horrors for the working poor in our country who are under very little labor protections; all of these issues would have been stymied by a union protecting labor.
At least with a corrupt union you have some recourse where you can drum up internal support among similarly exploited people, and change your organization via vote. As a nonunion worker, in contrast, you can't do anything to enact change if management isn't playing ball with labor, short of quitting your job and losing any and all your benefits like healthcare in the process.
1. https://irle.ucla.edu/old/publications/documents/LAwagetheft...
Wage theft is serious, but has enforcement paths independent of the presence of a union and if reform is needed, it should start there.
I am sympathetic to the plight of the poor but they can be harmed by labor practices supposedly friendly to them.
Sometimes people end up in the "yes I can do that" status group.
Could be they have union representation and contract rules that give them considerable agency coupled with low cost and risk.
Could be they are untouchable for some other reason too, principle friend, family, lover, child, sibling... Or, could be they have secured it through various relationships, power, leverage.
Unions do not cause this.
Poorly functioning workplaces can permit it to happen, and any workplace, union or not, can be poorly functioning for a ton of reasons, legal, contractual, managerial, financial...
Where that agency exists, sometimes people exercise it, simple as that.