I am a member of two unions. One is a cesspool of corruption and nepotism where the younger members have to pay up more than older memebers who make no effort to learn new technologies to remain competitve. Where its easier to go on strike than it is to train people to do their jobs better, and where millions "disapper" from the pension fund, where the former president built his second mansion in Utah on union dollars, against whom the DA of the city once had a case against for rackateering which was later dismissed due to bribery of his superiors, and where not toeing the party line can lead severe consequences whose punishments are doled out by a star chamber. In fact it reeked of the secretiveness and party line toeing that communism was known for.
My second union however asks a fair dues schedule, provides me with health care and pension, and will take former employers to court to get me more money if they are cuaght cooking their books, a notoriuosly common practice in my industry. Without them I'd be making half my wage and unfairly so, as my industry is very competitive and given to mistreament of employees due to high turnaround.
In short, it depends on the union, as in all things.