I've found even "normal" sized phones, like my S22, can encounter UI bugs where crappy interfaces appear to expect iPhone Max sized screens and so end up hiding bits of the interface behind button bars or fixed hight screens that hide the "next/submit" buttons behinds keyboards. I can only imagine how much worse these banking apps are on a smaller screen.

Sometimes they are just impossible to use. For example I could not hit the button to use Steam Guard in the Steam app to log in on one device. The screen was just too small for what the app expected.