Unions are democratically run by the workers for their own benefit. They should be mostly upside, or tradeoffs in your favor, since they're doing stuff that you voted for on your behalf.

People make a lot of noise about union dues, but they're a drop in the bucket compared to the higher wages the union will negotiate for. The most onerous requirement is being prepared to strike (and a lot of union dues are to set aside emergency funds for people during a strike).

That said, not all unions are equal. And like any small democratic institution, they're going to preferentially represent the people who get involved.