Conceptually, unions have succeeded historically to improve working conditions, to manage fair and honest promotions, job changes, etc.
I think first generation unions work quite well doing the above, because new unions don't really get started unless working conditions are bad.
I think where unions go wrong is in subsequent generations, when contracts get out of line, market dynamics are misaligned and everybody tends to look out for themselves. I like the motivations of unions conceptually, I just can't swallow how unions today have made things so ridiculous confusing and inefficient.
I've always worked in corporate office where the employees were non-union...I can see the appeal of a union for those marginal employees who would benefit greatly from being considered one of the masses. The truly productive employee typically loses out on greater rewards.