I'm already in 2 unions. Pros and cons, with the biggest con obviously being pay since now you have to collectively bargain and most union leaders are selected based on popularity and political ambition and not their negotiating skills, so your wages and benefits will go down though your overall job security will increase, and they will have to pay you overtime.

A pro is the work environment is much more honest, you are free to speak your grievances about management in front of everybody without worrying about them trying to fire you or scheme to make your life hell. You will have employees elected as shop stewards to handle all employee disputes instead of bringing in the heavy hand of management to deal with petty personal issues. Anytime you are summoned to a meeting room for discipline you get to bring a representative, and you get to appeal every single bullshit disciplinary action they take and almost always win, managers have no clue of the collective agreement.

Your job also changes to being a contractor, as you are contracted to do specific things which of course means if you finish them early, you can leave and still get paid 8hrs. You can also refuse work you have not been contracted for, and this is of course the major complaint about union employees how 'they're lazy and won't do what I ask' but if you asked the plumber you contracted to install a dishwasher to also mow your lawn for free they wouldn't do it either. That's how a collective agreement works, you have entered into an agreement to do X, unless there are special provisions for Y you don't do it. One of my collective agreements has provisions for extra work, and every day we push software on time in a research lab without some manager hovering around.