Yes, unions can and did greedy. That doesn't mean that they aren't valuable.
As more workplaces in the 30's and 40's became unionized, the middle class grew. A public and free high school education was enough to get you a decent job, a good work/life balance and a pension. Not only did that benefit the workers themselves, but those workers had disposable income that they could spend on other products and services, which meant more jobs for other people. These same people could afford to send their kids to college which fueled further innovation.
If an employer needed more production, there was a financial disincentive for them to have their employees work overtime, so they had to hire more people. These people also made a living wage, didn't require welfare, paid taxes, and contributed to the economic vitality of their community.
All in all it worked pretty well for everybody.
But employers are always looking for ways to cut costs and labor is typically a company's biggest expense. So then we got more automation, jobs being outsourced, and union busting. Big companies paying low wages came in to displace smaller ones "to create jobs". But the number of new jobs created is never as many as promised nor do they pay as well. Those jobs that do get created often get moved someplace else or cut altogether when the company merges with another one.
Today, outside of the few union jobs left, it takes specialized education, one you have to pay dearly for, to make a living wage. Graduating college students start out in huge debt and are encouraged to start saving now for their retirement because nobody has pensions anymore and social security is expected to disappear. Hopefully they can get a job that allows them to pay off their student loans before they need to start saving for their kids college education. And hopefully they won't find their job outsourced or outdated by some form of automation before they have saved up enough to retire.
I'm sorry, but I much preferred the outlook for joe and jane six pack when unions were actually relevant. And I'm afraid automation and outsourcing has started whittling away at white collar jobs too. How long before only the most specialized and expensive skills to obtain will be valued enough to pay a living wage for ?