Most unions demand standardized pay scales, maybe varying with seniority or experience. Slichter et alii (1960) say wage standardization is the most widely heralded union wage policy. Freeman (1980, 1984) showed that wages compress due to wage standardization at union shops.

Because wage dispersion and inequality is lower, if you consider your skills and experience extraordinary, you will not be getting pay commensurate with your perceived value to the company at a union shop. That provides a moat of sorts for incumbent workers against "rockstars," "10x," and higher-skilled workers deciding to join the company.

Slichter, S. H., Healy, J. J., and Livernash, E. R. (1960). _The Impact of Collective Bargaining on Management_.

Freeman, R. B. (1980) "Unionism and the dispersion of wages," _Industrial and Labor Relations Review_.

Freeman, R. B. (1984), "Longitudinal analyses of the effects of trade unions," _Journal of Labor Economics_

Well, in many professions unions demand that you only hire unionized workers. Hire one person person who is not a member, and the union workers will go on strike. Sometimes it even get's codified into the law, that you have to consult unions on everything.