IME, the UI library idiosyncrasies are the pieces that take the longest to get right. Common business logic is super simple by comparison.

> A huge amount of probably any app can be contained in a common blob of business logic and wiring, with some native presentation on top.

I keep seeing people claim this, yet most UI software I see really doesn't do anything that complicated. It's some basic database CRUD stuff and some basic algorithms, and then all the actual effort lies in making the UI feel good.