Unions are a double edged sword on the positive they do tend to protect against some of this stuff. On the negative they are expensive to maintain and that money comes out of the employee's check every week. Also they tend to rob you of flexibility.
My brother is a union member and because of that union contract there are some things he simply doesn't have to put up with. On the other hand because of that union contract he can't have alternative work schedules, he can't negotiate different duties with his boss, and in general he has far less flexibility than a non-union employee. My brother gets irritated because the union is dominated by a lot of employees who fear change so he can't get some of the things he wants because the majority votes against it. So realize if you accept the union you are agreeing to basically let your co-workers have a vote on your career decisions because those decisions will be made collectively rather than on an individual basis. Whether this is a good or bad thing is going to depend upon who you are, what industry you are in, whether there are better deals to be had and how likely you are to be able to negotiate one of those deals for yourself. If you are a top 10% employee for example odds are a union is going to be bad for you. If you are more of a midrange employee or someone who isn’t comfortable negotiating for yourself, and will thus never get those available deals, the union may be good for you.
Another aspect of giving up your own autonomy to gain the protection of a union is that you may be called upon to go on strike. This has happened to my brother twice in his career and both times it was a real hardship as their strike fund doesn't totally replace your paycheck. You also end up picketing when they tell you to even if it is raining in January.