Not tricky at all. The thing that worked out very well for Plaid in US was that the top say 25 Banks controlled more than 90% of the accounts held in American Bank (I’m guessing here). In Europe, that number is evenly spread out between the 3,000+ banks that they have. There never was a need to validate someone from across the border, but within the Eurozone as far as apps are concerned. Not to mention the language barrier for each country. No to mention the commercial demand has to be present.
A few companies are tackling this, slowly. figo | Europas erster Banking Service Provider is one such company.