> In terms of overall welfare
What is overall welfare ? If you are talking about society then we already know that free market competition leads to the best outcomes. If you are talking about welfare of specific group of people (at the expense of others) then you are right.
> 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 do not think anyone is opposed to that idea. It is covered under our freedom to form association. But in many cases unions are exact opposite of this. There is only one union which will negotiate on your behalf whether you like it or not and will actively prevent you from selecting someone else to negotiate on your behalf. In such cases workers are less free than before and it is much easier for the employer to get their way by just bribing the union leaders. (This is what happens in every single society where unions have collective bargaining rights.)