Have you ever worked for a union? It's just another entity that you have to negotiate with, except not only are they a self interested entity like your employer, but they also average out your interests within the context of their other members. It's good if you were on the bottom half of the distribution and you don't value independence/personal leverage. If you are a talented individual who is able to make your worth known to a self interested employer, such that you can typically negotiate higher pay, or choose to work on specific things, at specific hours, etc, unions can be extremely constraining. They often have arbitrary systems in place of meritocracies, where a position you might be better suited for, which your employer would prefer to have you do, will instead go to someone with seniority, or someone who has played the union political game better than you.

In other words, it's just another interested party that gets inserted as a middle man between you and your employer. If you weren't lacking in leverage, which most engineers are not, you're unlikely to find the upside.