In the US, it is illegal federally to mandate that an employee join a union as part of their employment, and it's illegal to force them to pay dues.
Some of our states go a step further and have introduced measures that seek to directly attack the ability of employees to collectively bargain, to remove the power organized labor has against management. These are called "right to work states".
Generally speaking, RTW states have lower pay on average, they are less educated on average, and their workers have less benefits from their businesses (parental leave, vacation time, healthcare benefits, etc), they have less recourses against employer bad behavior (both as a function of RTW attacking collective bargaining, and from the reality that the same politics that are against unions also are very anti-labor, anti-minimum wage, anti-regulation of labor marketplace all together, so there are far less public resources and regulators to assist with claims). If you're an employer, this probably sounds great. Labor is cheaper and you can fire them whenever you want for no reason at all on the drop of a dime AND there is no pesky government regulator to come waste your time and money investigating. "Easy come easy go" is one of the most common management philosophies from small and medium business owners in my RTW state, from my experience.
From the perspective of an American: you choose a liberal/blue/union state when you want good infrastructure, highly educated and competent staff, and little turnover. You choose a conservative/red/anti-union state when you're planning to take advantage of your staff, you don't care about higher turnover, you want to pay below-national-market rates as a rule, and you don't care as much about the individual quality of each employee. There's a reason why tech companies don't pick the South and when they do they require billions in literal free cash to do so.
It is what it is, but the slow death of the union in the United States correlates quite perfectly with the slow death of the American middle class in terms of wealth and income gains per year.