This doesn't sound like an advantage for software engineers. Surely unions can't decide on someone's competency. It kind of raises a red flag about potential gatekeeping methods (e.g. the tax status where you have to join or else).
Also disallows noncompete clauses. So if your current employer says no, you can go to another. Which is how I've seen quite a few promotions. The latter of switching employers is far easier, since life changing benefits (medical/retirement) aren't tied to employers. (Considering SV workers get their "share" by switching employers every few years, that would be a nightmare scenario for big tech as well. Since it further reduces employee stickiness, if SV unions decided to offer benefits).
But yes, there are gatekeeping effects, as the union is incentivized to prevent increases in membership or decrease in collective skill. It typically works out great for those in the union (and things like the Bar or Medical Association), not so great for those kept out.
I'd much rather have 8 companies with bad interviewers and 2 with good interviewers than 10 companies with the union who block me because I made one of their evaluators personally mad at me.
This just sounds like it is ripe for corruption and nepotism.