Testing could be much easier for the implicit state version. I've not defined it, but you are assuming things about how the render function or the implicit state behaves.

In my hypothetical implicit state framework - A widget is composed of a sequence of steps, each of which is an independent widget. So it's easy to write invariant properties for each step -

For (text "String") it's guaranteed that the widget never returns a value or raises an event. i.e. the type is

    forall a. Widget a.

So the issue of calling render a third time never arises.