Unions are very much an in-group vs out-group phenomenon (and in many cases, the benefits are specifically to the more senior union members vis a vis the less senior ones.)
In the US. In the UK all the major unions I know don't require membership and fight for all employees in the industries they represent. E.g. as a software developer on a research project at a University I got all the negotiated benefits of the academic Union.
If you're talking about AB 1751 (Missing Middle Townhome Ownership Act) I think you have this exactly backwards.
> Unions are very much an in-group vs out-group phenomenon
As opposed to what? You and the GGP phrase this as a dichotomy, but I'm really curious what the other side of this is, because "not being in a union" doesn't erase your self interest or all the many many groups of people doing the same that don't feel like they're apparently being overly self interested by forming corporations, governments, non-profits, advocacy groups, etc, etc, etc