Although this is not the case, moving away from proprietary services (and self-hosting your own) is an important goal in itself. See for instance the recent controversy regarding Discord's age verification.
https://privsec.dev/posts/android/banking-applications-compa...
This is linked to from the Banking Apps section on GrapheneOS docs: https://grapheneos.org/usage#banking-apps
Sample size of 1: my UK banking apps all work fine.
I've had less issues than with CalyxOS for example, where more apps broke.
https://grapheneos.org/articles/attestation-compatibility-gu...
https://privsec.dev/posts/android/banking-applications-compa...