No, unions are not inherently bad. They've done some necessary things, such as improving workers' lives.
Some people credit unions with preventing Communist revolution in the U.S. During the 1930s, some union organizers wanted revolution, but others wanted simply to improve conditions for workers.
Both FDR and Republican politicians supported the latter faction, and by improving worker conditions, deradicalized many workers. During the 1950s, the AFL (later merging with the more radical CIO) was an active opponent of Communism.
The Teamsters were supporters of Nixon and Reagan. They and the AFL-CIO both supported Solidarity in its fight against Polish Communism.
So while there are radical left elements within some unions, unions are neither inherently nor universally bad.
That's nitpicking. Unions will force members to pay dues for things the members do not support. Unions will force members to do picketing for other non-related unions. Unions will also force members into silence and not allow them to speak out about policies having nothing to do with the worker's rights. Unions are bad!
How do I know? In the past, I was not only a union member, but also a union officer and shop steward. I had a lot of disagreements with union leadership, many that simply wanted to line their own pockets and not work on actual worker grievances. They are bad, bad, bad.