The only thing I dislike is protecting the bad apples such as police union or teacher union style tenured.
I feel like company HR aren't there for you but for the company and unions are there for you.
I was in a situation were I was harass by my boss and HR was never there for me. After twice reporting, there was an incident where I decided I'm going to quit because I couldn't handle the harassment. This was in a gov agency mind you and I regret not joining a union.
We also had an option to take out of our paycheck a monthly fee for legal advice and retainers. I got that option just cause I thought it was a nice security I really don't know why I would need it. It turns out these retainers cannot help you legally against your boss or company btw.