How do you prove that an employee took two weeks to find a bug that should have taken one day to find?
How do you prove that writing a domain-specific language was a bad decision compared to defining a complex XML schema?
How do you prove that an engineer is bad at explaining technical concepts?
Email archives of PIP goals and the like. But that doesn't even matter. All you really have to do is prove you didn't fire them because they asked for OT pay they were owed.
I do think employees need organization and culture to negotiate better in the short term and manage health and career in the long term. But I don't see evidence that either the employer or the union leadership have the right incentives to meet those needs.