If anything, I've seen union doing exactly the opposite, at least in Germany for senior level jobs. Coming to work for a big company the only chance of getting a higher salary is to be above the union negotiated margin. The companies generally don't really want you to get there and it can be really tough to negotiate this. You are really well protected and basically unfirable in union but there is a top margin to your salary. If outside the margin of union salaries you're on your own with your own responsibilities. I really did not like working on a company that had a union as I've seen lots of people with zero motivation and nothing that a company could do about it.