Python on mobile devices is still negligible mostly because of the GUI problems which arise. But Python on android devices is considered to be a solved problem since the module Python for Android is available. We have nowadays at least the possibility to develop for mobile devices with the GUI engine Kivy which is maybe the first choice for rapid prototyping on that platform. But as it comes to design work and mobile device look alike Kivy is still far behind the web development tools HTML/CSS.
The advantage is to develop UX and back end code only once and use it literally on every platform where a Python interpreter and a browser runs.