This initial post is short and somewhat stream of consciousness. If it generates real discussion I'll probably expand on these thoughts at a later date, after I've had time to rethink things based on others' thoughts.
A union is neither intrinsically good nor intrinsically bad. If a union is out to help the employee, or foster a positive work environment for both employer and employee, or ensures an agreement is not completely ignored, it's a good thing. If instead it starts to strong-arm people, or act as an additional advocate for the employer (thus placing the employee at an additional disadvantage) or if profit is the primary motivator, or if the goal is to enforce an agreement without considering why the agreement exists, it's a bad thing. A bad union will push an agenda; a good one will try to genuinely understand both sides and promote a happy outcome for everyone.