When this was brought up in the past HN/Uber discussions, several commenters made the point that a union runs into much the same bad incentives, in that they need to protect other employees with more "value" to the organization -- which may include the harasser -- just as the company wants to defend high performers.
That linked comment is pretty ridiculous. Unions only have that kind of hire/fire ability in some of the building trades. I've worked for 4-5 different employers that were unionized, and they had nothing like that kind of influence. You work for the company, who has hire/fire rights, under a contract negotiated by the union.
There's definitely a bias towards protecting useless people, for the simple reason that divide & conquer is a key tactic for disrupting the bargaining position of a union, and it's always a better play to defend the dopes.
WRT to harassment, unions almost always help. The company has an interest in not getting sued, the union has an interest to protect the employees. As with any relationship involving humans, things go off the rails sometimes, but you are protected against most types of arbritary and capricious retailiation.
Characteristic example of the point: https://news.ycombinator.com/item?id=13686560