I see that point, which is why I'm not entirely against unions, but they have their downsides too.
Unionization has brought about some very beneficial improvements in working conditions and worker benefits and rights. But unions that get too big and powerful are often just as bad in different ways. They tend to make pay increases and promotions based solely on seniority instead of any measure of actual skill or work ethic, demand adherence to finicky rules about exactly who is allowed to do what work when, lose interest in representing individual workers when they don't fit the mold exactly, be just fine with driving business into bankruptcy or non-competitiveness with international competition, and generally foster an adversarial relationship between workers and management.
Corruption is equal opportunity. Police unions are known to keep bad cops around. Teacher unions are known for making it hard to fire bad teachers. I agree there are bad union implementations, but these seem like implementation problems rather than core idea problems.
In the list, just about every bad union behavior corresponds to an equally bad or worse corporate behavior. Bad behavior correlates with unchecked power.
The nature of workers and management is adversarial. Working for a company is trading your stress/time for money. The company tells you it's bad to share your wage because that creates upwards pressure on wages, while they go behind your back to ADP and buy your wage history[1] so they can pay you less than you're worth/what they budgeted to create downward pressure. That sure seems adversarial to me.
I do agree that international non competitiveness is a worry, but that's also an argument in favor of letting big un-regulatable monopolies run wild (the counter argument being that monopolies kill/acquire innovation). I suspect parents who work at sweat shops tend to raise more poorly educated mentally worse off children, which then hurts the next generation of engineers/innovaters and society as a whole.
I think I would be most worried about agents (corporate or nation state) agitating small disagreements into big ones or directly compromising and harming industry via agents emplaced in union leadership.
Unionization has brought about some very beneficial improvements in working conditions and worker benefits and rights. But unions that get too big and powerful are often just as bad in different ways. They tend to make pay increases and promotions based solely on seniority instead of any measure of actual skill or work ethic, demand adherence to finicky rules about exactly who is allowed to do what work when, lose interest in representing individual workers when they don't fit the mold exactly, be just fine with driving business into bankruptcy or non-competitiveness with international competition, and generally foster an adversarial relationship between workers and management.