How do these certificates compare against letsencrypt on the technical dimensions? e.g. certificate chain size, rate limits, whether every certificate is published to the certificate transparency logs, what OSs the root CAs are compatible with?
It sounds like the roots are the ones that Google uses to sign it's own domains (like Google.com), which are these: https://pki.goog/repository/
So likely support for them is fairly broad.
No longer especially that pre-LE, those same third-party sites already broke the rules, even with their EV offerings. Everything has been reduced to "is this domain at least controlled by them?" which is easily auditable (organisational verification has been significantly devalued). Now, Let's Encrypt only verifies domains and not trustworthiness (in fact, they won't revoke certificates known as phishing sites). Also EV certificates are nearly worthless unless you want to bypass many antivirus' HTTPS interception.
Why does it have to be a third party? My understanding is that as a user you just need to trust the certificate and it's owner, and as long as the trust is there it shouldn't matter what that certificate is signing.
The strange thing is that some companies mix and match - aws.amazon.com is signed with their certificate, while www.amazon.com is signed with a DigiCert certificate. You'll find Microsoft seems to mix certs as well, also using DigiCert for some sites. No clue why this is.