The problem is that Stripe requires sub merchants to create an account as well. Whereas Balanced, the account process was part of the API so your customers stayed in your application. That's going to be the biggest blow to the UI.
What are the reasons that you don't support Transfer API in Europe? We would need that but are forced to operate with a different model on a top of Braintree at the moment.
There are a lot of reasons, but the short answer is we're aware of the demand. Stay tuned :).