Sorry, I'll make the connection which was obviously unclear because I skipped a lot of steps. Unions concentrate power. Concentrated power is easy to take hold of (cf.
Coup d'Etat, E Luttwak). Lots of SWEs are incredibly xenophobic - can tell in any thread on HN and Reddit on H-1Bs. Xenophobes (of whom there are many) tend to be protectionist. Xenophobic protectionists are very good at holding power. Therefore, given sufficient time, probability of any power structure being taken over by xenophobic protectionists is almost 1. I don't want that. This industry flourishes because it is unbounded. No union. Will dedicate myself to scuttling the union if it gets any reasonable power.
Collective bargaining is a powerful tool. I recognize that and think it would be good for good people to have. But the danger is too high here.