I just assume that it's almost never going to be the case.

It might look like it's created by worker, but it's created by a sub group of workers which is unhappy with a status quo. This effort will be led by even smaller sub group of activists that have a drive and skillset to organise and weasel into a leadership positions.

Worker run union seems to be even more implausible. It's going to be run by a managerial body composed of said activists, which I absolutely expect to be self-serving, rent seeking and, well, parasitic. Occasionally their interests will coincide with interests of workers they represent, but that will be just some happy accidents.