If you've ever tried to do even the most basic of UI's in game engines, you'll quickly see the truth in that statement. Even "easy" things on the web like text

If you've ever tried to do even the most basic of UI's in game engines, you'll quickly see the truth in that statement. Even "easy" things on the web like text input boxes, scaling for different resolutions while keeping text crisp, and scrollable content areas are much harder in a game engine.