This was B2B SaaS for large networks of ambulatory centers to manage/pay their vendors. The banks that were in scope were only the ones they used -- each one of the big names (and around half of them Bank of America).
Can see it being more of an issue if it were a B2C finance app.
When we built our Plaid integration it used OAuth and a redirect. Plaid just got an access token, you enter your user/pass at bank side.
Edit: Seems like smaller/local banks are probably the ones that won't support OAuth. We didn't support those.