There's no such thing as platform independent UI, at least on that break. You can do a "mobile first" UI that sucks on desktop or a "desktop first" UI that suck

There's no such thing as platform independent UI, at least on that break. You can do a "mobile first" UI that sucks on desktop or a "desktop first" UI that sucks on mobile, or you can just make the decision to only support one first - nail it - then support the other, with a completely different UI. Sharing code is the easy part. The input and output mechanisms and paradigms are completely different, and designs that work great in one context are difficult or impossible to use in another (eg, key bindings or gestures), and hard problems exist in one area that don't in another (mostly on desktop, windows are terrible).