A union lets you have leverage when negotiating for anything with the corpo. Individually you have a little if you're top talent, and none otherwise. Very few people are irreplaceable, some are somewhat painful to replace, the rest are less so. We've been mistaking the tight labor market in this industry for our own self worth but hopefully the last couple of years have helped most of us snap out of it.

Speaking of pay, the structures I've seen at a union university for example have pay scales based on the job and defined pay increases in every job. You know what you're gonna get paid for a position you're applying, and you know what you're gonna get paid years ahead in that job. With that said, a union can negotiate any sort of pay scheme. Perhaps most importantly a union can negotiate to get a much larger portion of the profits for the engineers. You think some folks in tech are paid very well, but if you look at the value they generate, they might not be paid nearly enough. If you think a union might take your 500K salary to 300K while raising some other people's salaries you should consider that a union can take it to 800K or more. Assuming this is happening at one of the wildly profitable companies where this money exists.

And of course a union gives you the leverage to negotiate any other conditions like the ones that you mentioned. On-call, PTO, remote, etc.