By definition of function, employees and shareholders are at diametrically opposed incentives if the organization prioritizes return on capital over all other things.

If you are in an organization, where in the majority of the ownership is held by the people who have funded it, then at the starting point, it is already adversarial unless you are an equal partner in equity.

Since the vast majority of organizations are set up as such, and unless you have a controlling interest in the organization from a legal stock perspective, then you are in a position of no power to start with, and it will continue that way until you become a significant shareholder.

Unions are required to provide the collective action necessary to counter the overwhelming legal power of shareholders in the current structure.