My union has done nothing perceptible during the course of my employment, outside of protecting employees who are under performing.
My union dues cost 1% of my salary, and my perception is that they're taking that money and spending it on protecting bad employees.
I have friends who are higher up in unions at other workplaces, as well as friends who are involved with labor law. The impression all around is that my union and its representatives are "not good".
Unions don't automatically make working conditions better. They need to have good people working in the union to achieve that.