One difference is how they gain leverage. Labor unions seem to rely on the fact that they are the ones _right there_ who can work in a factory, clean a facility, etc. But capital sort of did an end run around some of that by literally packing factories up and shipping them overseas. That seems pretty easy to do to knowledge workers as well. At least to some degree and at some level.
This is far outside anything I've read extensively about, and it'd be interesting to read more.