Not true. I'm a customer of 3 top French online banks and of two top global brokerage cie and the apps work great. I have no issue whatsoever except with one health insurance app but their Web app is better anyway.

Only thing missing is contactless payment but there are European banks that support GrapheneOS without Google Pay so alternatives exist. And I can use Garmin pay with my watch (and offline and without Garmin Connect thanks to the FOSS GadgetBridge appp) if that was necessary.