For me, the showstopper would be NFC payments. From what I understand, Google Pay doesn't work on Graphene. I have all my credit cards in GPay, as well as a transit card. I use it for boarding passes when I fly, and any other tickets/passes that support it, since it tends to be much more reliable than the airline or ticketer's app. I've come to heavily rely on it, unfortunately.

I haven't tried this, because I try to minimize Google exposure, but I think Google Wallet (minus NFC payments) works on GrapheneOS. So, tickets, boarding passes, etc. should work fine.