If you talk to teachers who worked in the pre-union days they'll tell you it was a nightmare. Because it was run by low-level politicians (school board members) who were cowardly to the point of causing havok.
Put it this way. Most people don't know the political positions of their individual school board members. So if someone comes to their house and says "this member is bad" they'll generally vote against them on the theory of "better safe than sorry". Which meant every crazy parent who thought their kid was treated unfairly would raise a fuss and get teachers fired because the Principal answered to the School Board members and those members had extremely tenuous holds on their position. Meaning teachers were getting fired fairly regularly, often for doing nothing wrong.
Teachers Unions were born out of that abuse. The problem now is we've swung to the other extreme. Where even teachers who are bad can't get fired. But I wouldn't say the problem is the union contracts. If anything it's that we haven't found an appropriate metric for measuring teacher performance.