In general it's better to be in a union than not. They arn't perfect, but they at least add an obstacle for the company to take stuff away from you and they also keep certain order like seniority, pay etc. Without unions employers can do 100% whatever the hell they want, such as forcing you to work overtime, or firing at will. Unions require a due process for termination, it has to be legit and documented etc. Of course unions differ between companies so some may or may not actually protect you.
My company is so big that in all reality they can pretty much do what they want and the union does not have much say. When it comes time for contract renewal they usually take stuff away from us, and if we were to oppose it by striking, they'd just close us down, so they kind of have us by the throat even though we have a union. We lost our bonus and cost of living pay increase so far, along a lot of benefit related stuff that affects new people. Pretty sure this next agreement they're getting rid of the retirement plan. What are we suppose to do, apply for a different company? *south park scene where guy rubs nipples* "What other company?" When the job market sucks employers can do what they want union or not, but union at least makes it slightly harder for them.
One thing I hate about non unionized places is that your pay is not based on the department you work in, but is based on your negotiation skills during the interview. With a union there will normally be some kind of step program and once everyone reaches the cap they make the same, with exceptions of people who might be grandfathered into a previous agreement. Though I guess this can differ from company to company.