I can only speak for the union in the company I work for.
I was in a unionless company before. Than a merge with another company happened and suddenly we had a union. One of the first things they did was unifying the work contracts. We got a 13th month payment every year before the union. With the new contracts the payment was higher, but we only got 12 payments. The overall sum didn't change.
However they also altered our business trip conditions. Before the union 100% were paid. After the union 100% were only paid, if specific conditions were met.
Don't want to go too deep into the details, but to make it short the business trip conditions are worse than before.
So in my case the union made things worse. Our union also used some dirty tactics at the supervisory board election.
I still think that unions did great things in the past and there are probably still jobs and countries which need them.
I also support the union if they call out a strike. (They have a point, if they say they can't get good results without support.) But I won't become a paying member. Our union just did too many stupid things, which I can't forgive them ...