> The ethical side is up to you, but in a strictly technical sense I don't think there's much that Google could do to intrude on your users privacy as a result of them issuing your SSL certificate, even if they wanted to.

I'm not sure that's technically true. As a CA they definitely have the power to facilitate a MitM attack. They can also issue fraudulent certificates.

> AIUI the ACME protocol never lets the CA see the private key, only the public key, which is public by definition anyway.

That has more to do with HTTPS end to end encryption, not the protocol of issuance.