https://github.com/WICG/floc describes something open source and running on the client. Will that be sufficient, or is there additional disclosure you'd like to see?
It enhances the ability to fingerprint me, effectively exposing my browsing history.
If Google offered it as an "opt in", giving me some reward for sharing my personal information that Google sells to advertisers, then that is a fair deal. In return for some form of sharing the revenue, Google gets to sell my information.
But that's not the model. It's still the "you're a product" model where Google not only gets to sell my search history, but now also continues to sell my browsing history.
I can see how it benefits Google and how it gives them/you something to sell to advertisers, but what's in it for me?
What you get in return is that ad-supported sites you visit are better funded because they can show better-targeted advertising.
The devil is in the detail. So if FLoC and new third party tools to subvert FLoC became too mainstream, then I would expect Google to act in its own interest and provide value-added back-end services. Just as has happened with Android AOSP and Play Services.
Until then though, I feel FLoC being both client-side and open-source would be an improvement on the status quo
(Disclosure: I work on ads at Google, speaking only for myself)