Good point, I'd definitely hate having to pay optional union fees but I'd love to pay mandatory lawyer fees when I try to sue for wrongful dismissal, instead of the union paying for it. Putting money into a pool that pretty can arrange for external HR (which could've avoided this whole KickVic nonsense) or helps me petition for better pay and work safety seems like a better idea than America's "yeah, it's up to the employers and if the employees don't like it then sue me" method.
As far as I'm aware, there's nothing forcing you to join a union in the EU, you can just go it alone.