As domh mentioned, some (not all) banking apps do seem to work well at the moment. My concern would be that what works today may not work tomorrow. My HSBC app seems to get more crippled with every update and it wouldn't surprise me at all if a future update rendered it unusable on GrapheneOS (which is the main thing stopping me from moving to it).

It's probably a pipe dream but I do hope that someone like Motorola officially supporting GrapheneOS will make businesses take support somewhat seriously. If nothing else you sound less like a crazy person when you tell your bank's customer support "I bought a Motorola phone and now your app doesn't work" than "I flashed a custom ROM to my Pixel and now your app doesn't work".