I don't think it's valid to write off all unions based on anecdotal evidence. For every example of bad unionisation one can probably provide one or more counter-examples of good unionisation (e.g. the Scandinavian countries).
In my personal anecdotal experience, the unions at the places I've worked have been mired in factions. They supported some workers but not others because they picked sides. They weren't just neutral, they undermined some workers to benefit others. It really soured me on them.
(My only knowledge comes from watching The Irishman)