Those are valid concerns, though at first glance it doesn't seem to me like any of Google's actual proposals[1] have those issues. In fact, FloC seems like a rather promising way to achieve "personalizing content without identifiable tracking". Perhaps that particular problem isn't as hard as you thought?
Floc is still a surveillance machinery that comes with the usual bells and whistles like mass propaganda and behaviour control. They admit to as much in the concerns section of the readme.
The concerns section explains that broad interest categories (traits shared with thousands of other people) are revealed to ad networks. That's it. No "surveillance machinery", nor is there any mention of "mass propaganda and behaviour control".
> A flock could be used as a user identifier. It may not have enough bits of information to individually identify someone, but in combination with other information (such as an IP address), it might.
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> A flock might reveal sensitive information. As a first mitigation, the browser should remove sensitive categories from its data collection. But this does not mean sensitive information can’t be leaked. Some people are sensitive to categories that others are not, and there is no globally accepted notion of sensitive categories.
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> This API democratizes access to some information about an individual’s general browsing history (and thus, general interests) to any site that opts into the header. This is in contrast to today’s world, in which cookies or other tracking techniques may be used to collate someone’s browsing activity across many sites.
> Sites that know a person’s PII (e.g., when people sign in using their email address) could record and reveal their flock. This means that information about an individual's interests may eventually become public. This is not ideal, but still better than today’s situation in which PII can be joined to exact browsing history obtained via third-party cookies.
[1]: https://www.chromium.org/Home/chromium-privacy/privacy-sandb...