I think some of this perception is due to many people on HNs primary experience with union jobs is through government employed unions (e.g. public school staff) where there is essentially zero wiggle room for pay; it's a simple algebraic formula that you can calculate yourself. They also seem to be loath to fire people at all, to a degree I haven't seen in e.g. factory union jobs; I think the whole "If the guy next to me fucks up, I could end up injured" helps in those cases.