The only major inconvenience I've heard of is that Google Pay won't let you enroll contactless payments via NFC, apparently because GrapheneOS isn't approved/certified by Google Play SafetyNet.
Google Pay doesn't allow using a non-Google-certified OS. It checks this with the Play Integrity API. It won't be possible to spoof in the long term, so we don't pretend to be an insecure old device without hardware attestation to trick the checks in the short term.
You can use a Pixel Watch or Galaxy Watch paired with GrapheneOS and make payments from the Pixel Watch.
You can use a Pixel Watch or Galaxy Watch paired with GrapheneOS and make payments from the Pixel Watch.