RN components use Yoga,
XAML elements use XAML (Measure/Arrange)

There isn't a way for XAML to be aware of Yoga (and the mechanisms for how layout works are incompatible).

So if we have a native panel that lays out its children and we try to add a RN component, the component won't lay out properly:

<StackPanel>
   <Text style={{ alignSelf: "flex-start" }}>foo</Text>
</StackPanel>