Essentialy banking apps hate unlocked bootloaders. GOS (GrapheneOS) avoids this relocking the bootloader (the key is theirs, if you want to build your own GOS you'll have to sign with your own key). However GOS still fails Play Integrity checks: it fails CTS Profile Match.
So, Banking Apps probably work but Google Wallet won't.
Additionally, they run Google Apps as non-privileged apps, using a compatibility layer called `gmscompat`. It's cool because it's easy to Degoogle your phone in an instant if you wish to. But certain niche features, for example, using your camera to help Google Maps match your surrounding with Street View data crash Maps.
Otherwise all runs mostly well. Waze a few weeks ago was wonky but I assume the bug they fixed in Wifi-location allowed Waze to behave -- haven't tested though.
I refuse to use my phone for banking due to trust issues in putting all my eggs into such a stealable, forgettable-in-a-taxi, and heavily monitored device basket.