The concern from people like myself is that another word for a union is a cartel. When companies form cartels and engage in anti-competitive behavior, we penalize them severely (in theory at least...but that's another issue). Yet when labor colludes, we simply call it a union.
Tech is especially interesting because the usual claims of "workers have less power individually" (which is always true in all industries) is really really not a great argument in tech. The labor market in tech is so unbelievably competitive, and the average worker has leverage that is only seen in the upper echelons of other industries.