In a post Lochner world, there really isn’t a need for unions to protect the basic health and safety of workers. With Lochner being effectively overturned in the late 1930’s, Unions are no longer needed in the U.S. There are now plenty of federal and state laws protecting workers (OSHA, FLSA, Title VII, ADEA, ERISA, ADA, etc.)
There is a long history of Union corruption in the U.S. Even those that are not overtly corrupt tend towards a level of bureaucracy in which the value of dues paid by members is less than what they get in return. There are always a few people at the top of union management which tend to get rich at the expense of the workers. Trade Unions have also proved fairly ineffective in managing pension funds.
In the U.S. (and even more so abroad in Europe), they tend be a drag on the economy.
Violence. There is a history of violence by unions in the U.S. This hit close to home for me as my grandfather was murdered due to his role in a union organizing campaign in the 1930s in Mishawaka.
Flexibility—In industries which require rapid innovation, unions can slow this down.
Meritocracy—Unions tend to value tenure over merit. I just prefer a system where the best performers are rewarded. This is particularly problematic in environments like public education.
I think unions had a place in the U.S. economy during the 1800’s, but much has changed since then.
I do support the unionization requirements of the USMCA. In summary it requires Mexico to implement unions in a manner closer to way they exist in the U.S. This will bring some long overdue protections to Mexican workers. (It will also in the long run be a step in closing the gap between the U.S. and Mexico in labor costs.)