Empirical argument: The biggest unions in america are for the most poorly run and corrupt organizations. Teachers, Police, Firemen, Government workers, Health, public transportation. All with massive costs to the consumer, corruption, service disruption, closed shops, etc.

Microeconomics argument: The only meaningful way unions increase employees wages is by artificially restricting supply. They can only move the point of equilibrium of demand and supply in their favor by restricting supply, thus unions always have in their interests on: increase minimum wages, licensing, closed shops, etc.

Political Economy argument: What is most interesting for unions is to survive as unions, which means that even if sometimes they are adversarial with employers, they are often allies to the structure. This is because at the negotiation table between unions and employers there are some actors missing, namely, the consumer.

Political argument: Unions exhert force only by destruction: by refusing to work, by boycotting, by collecting administration fees to maintain its structure. The proliferation of unions is the proliferation of destruction. Giving special legal status, protections, considerations, rewards to unions, you are proliferating unions and proliferating destruction.

The arguments against come from uncertainty, "there are clearly trade offs" "I don't know but people with skin in the game are against it" "This will cause problems in the long run"

The arguments in favor are certain. There's is no mention of downsides; the only talk is of more money and more security with at most a mention that some dues will need to be paid (though on balance the worker will come out ahead).