Bold move with the switch away from Ember to a lesser known framework. I'll admit that makes me a bit worried, considering a framework like Ember is sure to be maintained and supported for the next 5-10 years, while Mithril is not.
Still, if it significantly improves your extensibility story, you must have had your reasons. It'd be interesting to hear in greater detail what those were though. Could be useful for the Ember community to hear about as well.