My experience with unions is that they start out with noble intentions, and then within a generation or two of union leaders devolve into just serving themselves; meanwhile, management suffers with worse and worse employees, because they can't do anything to get rid of the bad apples. Over time, the bad apples bring in more bad apples and things get progressively worse. That can happen in a non-union company, too, but then it'll get punished by the market.

It's really a question of misaligned incentives; the union reps have no incentive to do anything to help the company, and often have incentives to do the exact opposite, like initiate a strike at the worst possible time.