I've been dailying GrapheneOS for more than 2 years. I haven't had any issues with any of my bank apps. Although there are apparently some that do block usage on non-google-certified OSs. The only limitation in my use cases pre-GrapheneOS that I've found is not being able to make NFC payments through Google Wallet, but I found a bank in my country whose app implemented NFC payments, and made an account with them to be able to convenintly pay with my phone.

edit: and I'd like to add, GrapheneOS brought me back the joy of using my phone. Since 2018 or so I started to dread my phone (and the internet) more and more. Installing GrapheneOS brought back the joy on using these marvelous computers (and self-hosting brought back the joy of using the internet)