[SpicyLemonZest]

The details of the initial organization aren't public, but it's likely that it went in the other direction, as part of the pre-existing project CODE-CWA trying to convince software developers to unionize with them. There's no reason I can see that they would have had to join the CWA.

The advantage of affiliating with a larger organization is their weight becomes a part of your collective bargaining strength. That is, if Google does something the union doesn't like then the entire CWA might get mad at them. I don't know that there are any clear disadvantages from a union's perspective, which is why basically all organizing efforts do it.

From a bird's eye perspective, the disadvantage is that there's no meaningful competition or innovation, because all new unions see themselves as part of the traditional union movement where solidarity is prized. If someone else formed a competing union with a clever new idea for how to organize Google workers, the CWA-backed union would denounce it and demand that Google refuse to talk to the second union.

[scarmig]

Disadvantages from a union perspective:

1) a substantial part of your dues are passed on to support the larger organization

2) some member services are delegated to the larger union, and some larger unions are better at member services than others

3) some larger unions spend a lot of money on political activism instead of member services

4) less independence of action, as larger unions might have different priorities than what ground level members want (e.g. wanting to get a contract settled instead of fighting for more; external organizing over internal organizing)