1) you have smaller screen estate and have to split some forms into multiple page wizards ( on desktop if you lack space you just popup yet another window wherever it popups)
2) you have to support different screen orientations (portrait, landscape)
3) you have to support corners cases like notch or foldable screens
4) you have to support different screen sizes and different screen aspect ratio (on desktop still many apps don't allow resizing and its ok)
5) you don't have precision input such as mouse where you can hover on some small button and see via tooltip what it does.
6) you don't have physical keyboard instead a virtual keyboard that on appear eats almost half of your already screen estate and covers other form controls.
7) mobile OS make limitation what you can do in the background, how much memory you can use
8) mobile OS don't allow just dragging windows and resizing, and switching windows easily using native UI