Earlier quoted context omitted.
At best, they do what the majority of their members want. Which may be completely at odds with your interests. For example a union of mainly older members voting for pay-by-seniority and damaging younger people’s pay. Democracy in a union can become a tyranny of the majority. That’s one reason many people don’t want unions.
Isn't that democracy everywhere though? And it's still better than any of the alternatives by a significant margin.
They are deemed unlawful when you force people to join, teachers freedom of speech was considered violated when they're forced to join but disagree with the demands of the union. Is it fair to be forced to join, your dissenting opinion ignored and pay people who work against your own interests?