> a) every site you visit loads some Google/Facebook/whatever JavaScript to track you. That are not all sites.
Right, but it is, to a first approximation, all of the sites that are going to be showing you personalized ads.
> Another bad thing about this system is that is integrated inside the browser, meaning that for a closed source browser like Chrome only Google knows how it works and what exactly it does. While classical tracker scripts that uses third party cookies are implemented in JavaScript, minified and obfuscated, but still you can in theory read the source code and understand what they do.
I'm not quite sure what you're getting at here. The setting and reading of a cookie by a client, yes, will be written in JS. That doesn't mean that you can know what its doing (there are likely cookies on your system that contain encrypted payloads that you can't read). The system that "actually" reads those cookies is hidden behind an API, so you don't have access to even the binary code.
From that perspective, FloC is no worse, and is usually better, as the payload itself is generated and managed on your machine.
Tangentially, FloC is similar to but probably more privacy preserving than the way Brave Browser does advertising today, and that was heralded by many on HN as a huge privacy improvement.