I've thought about this a bit. I don't think we'd get much public sympathy from a working conditions standpoint- generally if you're working for the right place you can arguably make more money than most of your non-engineer peers (look at a percentile breakdown of your annual income if you don't believe me).

I think the way a union like this could succeed is by fighting Big Brother-esque projects and refusing to work on initiatives such as Project Butterfly. Sell it as "we refuse to work on technology that will objectively make you (the layman's) life worse". Once we hit a critical mass we'd have a much bigger say over what projects do and don't get built, and anyone who scabs is arguably on the wrong side of the moral coin or "enabling Big Brother/technocratic billionaires".

This isn't to say that tech workers should be the ultimate moral arbiter, but having a non-financially driven check would at least help.