UI Designs are the end product, but before you get there, there is a lot of work to do. Many designs can 'look good and clean' but lack
best UX Practices, which makes the user's descision making harder, and the speed they interact with the UI and get the screen task done, slower.

 

On the left, you will see bad UX practices that many mobile products have.
On the right, you will see the best UX practices that speed up interaction between the UI and user and avoids potential user confusion and UX defects.