Same-Site cookies are, well, same-site. Not same-origin. This is already a deal-breaker for many deployments, because they don't trust blog.example.com and part

Same-Site cookies are, well, same-site. Not same-origin. This is already a deal-breaker for many deployments, because they don't trust blog.example.com and partner.example.com as much as admin.example.com (both in the strict sense of trust, and in the senso of not having XSS vulnerabilities the attacker can pivot off). Worse, by the original definition http://foo.example.com http://foo.example.com and https://admin.example.com https://admin.example.com are same-site, and unless the site uses HSTS with includeSubDomains, any network attacker controls the former. Chrome changed that with Schemeful Same-Site in 2020, but Firefox and Safari never deployed it.