There is right amount of give and take in all scenarios. If unions have too much control they don't get fired for doing a bad job, don't work that many hours, and become a cost burden for the employees. If the employees have too much power you get a situation that the United States faced in the mid 1800's.
I'll agree with there is a correct balance, but I can't say that our society has found that correct balance which we see as the "ideal" system. When mega corporations are purporsely working people just under 40 hours to avoid giving health benefits or not paying them for overtime and they can't form a union to fight this action, there is a problem. Let's see who can guess which organization i'm talking about with these dirty business practices. Here's a hint, they're trying to build one in pretty much every community even though most communities don't want it becaues of what it stands for.