Although many people do say that there's an issue with banking apps on GrapheneOS, I don't believe that's actually the case. Many banking apps work just fine. In cases where other apps refuse to run, GrapheneOS has compatibility toggles. After toggling those on, many banking apps that complain start working again.

In other cases, banks add Play Integrity checks to their apps, which is a more complex subject. In short, many banking apps check for the basic integrity verdict, which GrapheneOS can get. In other cases, the checks are not implemented properly, and GrapheneOS has a toggle which often gets those apps to work. Finally, a very small number of banking apps require the strong integrity verdict and the apps will refuse to run on GrapheneOS.

Although there are no statistics about this, it does seem that a majority of banking apps work or can be made to work with different compatibility toggles.