> I think that a union is just an entity that represents the workers.

In theory, yes. In practice, a union brings with it economic incentives to do more than just represent the workers. Many of its incentives are in conflict with the wellbeing of individual workers, the company, and the economic wellbeing of the jurisdiction in which they operate.

> What makes you less optimistic about it?

Corruption, bureaucracy, institutional inertia, incentives for rent seeking, and loss of agency for the individual worker.