another issue for businesses is that you pay lot more per transaction or batch when you use the api services of banks. For example we pay 2.50 euro for a sepa batch via ftp. For the same batch via the api we pay a monthly fee of 500 euros plus 4 euros per batch.
The api provides instant payment processing. The ftp is a daily batch processing.
We still use the ftp way, because the api one is too expensive
Then came all those new app-based banks like N26, and instead of implementing FinTS, they decided to just offer their proprietary app, and that's it. Some later implemented some kind of proprietary API as an alternative to FinTS (maybe FinTS wasn't hipster enough, because it's a bit old-school, XML-based instead of another REST-style HTTP thing with JSON), but those always lacked the core feature of FinTS, which is broad support of the same standard by many banks.
And now we're getting PSD2, which in some ways wants to establish broad (mandated by law) support over many banks again, but not by specifying an API designed to be used by customers, but by businesses only. Hence what we in Germany were able to do as customers for a long time - magically access dozens of accounts at different banks from a single application and without ANY middlemen involved - is now gradually becoming an ability that only corporations will have, because as a private individual it's practically impossible to meet the requirements to get access to PSD2 APIs (even though they basically do the same as FinTS already did for decades), and banks are already toying around with the idea of shutting down their FinTS API gateways somewhere in the future, because that other protocol is legally mandated and FinTS isn't.