There's a wide spectrum in "a union" just like in "management". I've had good management and bad management. There are good unions and bad unions. So, a priori, I don't have an expectation of whether "the union" - some arbitrarily picked union - will definitely help or hurt, any more than I have an expectation that arbitrary "management" will definitely help or hurt. But there are a few things:
- Just like I evaluate management when finding a new job (in my most recent job hunt I quite explicitly asked my interviewers what they thought about their management and how they thought it differed from other places they've worked, and hearing poor answers from one famous company was a reason I turned them down), I can evaluate the union when finding a new job. If I don't think the union will force management to deal with brilliant assholes, or properly equip people for success who have aptitude but the wrong background, or acknowledge that poor management is the reason certain of my colleagues are underperforming, I won't join the company.
- I have much more of a democratic voice in my union's priorities than my management's priorities.
- Management often prioritizes their own needs as individual employees, and is willing to overlook failures or mistakes if it's in the interest of their own career paths. It's hard to crack down on people failing up into management if you yourself are a manager because you failed up.