In terms of overall welfare, an additional dollar in the pocket of a typical union worker is going to be more welfare enhancing than that additional dollar in the pocket of the typical owner. This is from just basic diminishing marginal utility of the dollar.
Of course, unions can do awful, evil things - they are organizations of people and people are petty, self-centered, greedy, etc.
I'm in favor of workers' ability to elect someone to represent them at the negotiation table (because that is what a union is). I'm not unaware that sometimes those workers can elect bad people.
I'm also in favor of electoral democracy, even if I wouldn't personally defend every single person who has been elected president since the start of time.