If you have access to an API to process BACS transfers, you might be able to avoid having to pay percentage VISA or MasterCard fees. For any non-trivial sums of money, Griffin's fixed-rate pricing is rather competitive. The only real downside of transfers at the moment is needing the human in the loop, whereas card payments have already had fully automated platforms such as PayPal, Stripe and Opayo for many years.
What user benefits are unlocked with seamless API access to a bank? Is it "just" something like Mint, but implemented in a more technically legit way?