> I can put together a full HTML/CSS/JS UI for a screen in hours

Hours for a single screen ? Exactly what I'm talking about. The fact you think that's quick is worrying.

You are the second person to say this, what are you doing that's faster?

I'm talking from start-to-finish creating the UI, wiring into business-logic, handling UI state, making it fully responsive, etc...

I've used WPF, QT, some old java swing stuff, and the Android studio layout editor, and none of them are a fraction as fast and capable as a well setup react web application (which to be fair, took a good chunk of time to setup).

Sure, because the browser will go out of its way to render something sensible out of any tag soup thrown at it, as opposed to throwing an exception like native frameworks do.

A competant iOS or OS X developer can use Interface Builder to create great native UI’s in minutes.

If you are including the wiring of the UI into the application in that time I'm impressed, however you still will need to do another 3 major platforms.

And you'll need to have a designer look them over or design them in the first place for each platform, and write documentation for each platform, and test each platform, etc...

In many cases I won't argue that a native app won't be better for the user in many ways, but you can't argue that it's faster to re-create, document, and test your UI 5 different times.

I’ve found it faster.

Faster to build 3-4 native UIs instead of one HTML based one?