What people are rightfully scared of is that:

• Big websites will only accept attestations from big companies like Google, Apple, and Microsoft
• Google, Apple, and Microsoft will refuse to attest your browser if you have an adblocker installed, or if you are using a browser or operating system they don't approve, or if you made modifications to your browser or your operating system etc.

While adblocking can be detected, you can block anti-adblock scripts, it's sort of a weapons race. Depending on how deep an attestation goes, it might be extremely difficult to fight. Attestations might also be used to block more than just adblockers, for example using Firefox, or rooting/jailbreaking your phone, or installing an alternative OS might make your phone ineligible for attestations and thus locked out of a lot of the internet.