One User Interface does not fit all.
The Flowee Pay design follows a very rigorous separation of model and view, following the designs earmarked by the QML programming libraries. This separation keeps the complexity in the C++ parts, and that means the UX designers have less to worry about.
The final result is that it takes much less effort to create and maintain multiple front-ends, while still being able to share most of the difficult parts between them. As Flowee Pay gets more feature-complete we expect there to be multiple front-ends that will be aimed at different audiences, or simply at different form-factors.
The development front-end (or user-interface) is the desktop one. If a new feature is added, you can find it here first. It may not have the best user experience, but this is mostly because developers write it and use it.