Leave the more general, "politics" side of unions (calling for general strikes, and so on) to CCOO and UGT, and make use of another union to protect you when your company tries to abuse you. Also, if you're not satisfied with what the unions do, you can always the works council. A workmate of mine is in our company's works council and she isn't even part of any union.
Also, in my experience CCOO and UGT are not nearly as bad as the media paints them (again, I'm talking about what's going on at companies I've worked at). This may sound like a cliché, but really, there is an awful lot of propaganda against them because there are plenty of rich people interested in having an exploitable work force. The only real complain I have against CCOO and UGT is that they sometimes don't fight as hard as they should. But in many other cases they do. I've seen cases where these unions have successfully sued companies upon wrongful termination of workers, for example.
https://www.google.com/amp/s/www.newsweek.com/why-are-people...
The main point of my comment, which I made very clear, was that specific examples of "bad unions" is not a helpful or constructive basis upon which to have an argument about the theoretical place of labour unions in a market, no more than you would base an argument about where to go for dinner in the nutritional value of a carrot.
How can you possibly guarantee (no less!) what someone, regardless of where they come from, will say about any subject? You cannot complain about generalizations and broad statements, as in here:
>...a century of propaganda has convinced most people not only that poor people are lazy, but by extension that unions are evil, corrupt and lazy organizations infiltrated by organized crime.
And then try to fit reality into your own narrative at the same time without being a hypocrite.
I will tell you this, though: CCOO and UGT (the two major trade unions in Spain) are publicly funded, which I find absolutely wrong. Moreover, they tend to be incredibly incompetent and useless in most cases. Of course, I'm drawing from my own personal experience and others' accounts of their own.