One day I receive a request that says you are group 12345, in 1 hour you are 12346 instead because the algorithm recalculated your FLoC... Not sure what anyone can tell about that without knowing the group ids meanings, which likely not even google will know since this is going to be a clustering algorithm anyway.
As long as browser fingerprinting exists it will be possible to track the FLoC cohorts a person belongs to over time and learn much more about them than a single FLoC cohort reveals.
But AFAIK, FLoC is only replacing third-party cookies; browser fingerprinting will still be possible.
Effective fingerprinting makes everything impossible. Even if there were no 3rd party cookies and no FLoC, ad-tech can still use fingerprinting to bucket you based on your browsing history. This isn't a criticism of FLoC, this is a criticism of unwanted fingerprinting.
Sites that know who you are (e.g. because you log in) gain even more info, if they previously only knew about your visits to that site.