prototypes on Ui stage

The prototypes on UI stage are created more for presentation of application general looks than for testing and improving its functional features.
And this is the trap in which it is easier to be confused. Prototyping all the details on the final stage of UI in most cases is not so reasonable as it could seem.
It will be too time-consuming and in this perspective, it would be better to spend the same time on coding a demo-version.

Moreover, usability should be thoroughly checked first of all at the UX stage, otherwise it would be much harder to change inefficient solution after having accomplished a lot of work on UI.
 Certainly, it would be amazing to create prototypes both for UX and UI, but by far not all the designers and customers agree to spend so much time on design tasks and want to test and improve the design much faster and cheaper for better user experience.