Denmark, Finland, Iceland, Norway and Sweden. None of these nations have an minimim hourly wage enshrined in law. Instead many of the base terms of employment, including wages, are decided via collective bargaining between sector trade unions and representatives of public sector and business interest organizations.
I live in Sweden by the way, so feel free to ask me questions on the topic and I’ll do my best to answer.
Not much that I know of aside from a bunch of extra bureaucracy. Unions already perform polls on wages amongst members, and that information is generally available to the union members. Furthermore, there’s often even information available about wages within an employer organization if it is large.