if unions are a good thing then why would hitler, for example, shut them down?
I think you just answered your own question.
Research how working conditions were 100 years ago and compare them to today. That will answer your question.
Yep. While there's certainly a danger that unions can get too powerful and lead to scenarios such as the UK being basically brought to its knees by militant unions in the 1970s, going to the exact opposite extreme and banning unions entirely (or rendering them powerless) would be just as bad, if not worse.