(We had layoffs unrelated to the union and more related to a bad monetization scheme and high costs, during which the union negotiated increased severance and health insurance coverage vs we were initially offered.)
Union successes are definitely growing in tech.
A more subtle "downside" is that unions represent the workers—including the workers you _don't_ like. That project manager who you hate is also a laborer with rights who deserves protection from the union, and at some point you will need to come to terms with cooperating with people you think have horrific political views or whatever. I use quotes because I don't think this is a real downside at all. Unions are not simply a cost-benefit option you choose because it helps you personally, they are the counterbalance to the implicit power employers have over their employees. Even if your benefits are great and you're paid very well, you should still have a union to represent you.
For union members pay, layoffs, etc. are often decided by seniority and not by ability or how hard you work.
Whether or not that's worth it is up to you.