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
I know it's not how the world works, but I wish, if we could just show "them" how good the programs can be, how efficient the user interfaces can be, how awesome it is to use a real proper computer with real proper computer programs, instead of the toys on tablets... But I know I'm in the wrong.. I know it's "just me" and everyone else enjoys their glass plates. grumpy old man shows himself out
Worrying about making it work in mobile is an external requirement that arises due to the world we live in, not because of funsies. If someone could say "fuck supporting the shitty options" without bleeding a metric-fuck-ton of money for it, they would
I assume you mean any significant amount of time. I often write something quick on mobile, which works okay. However if it is more than a sentence or two give me a keyboard. The mobile is with me all the time though, my nice desktop with a full sized keyboard, large monitors and all that is back at home. Thus the compromise of often using mobile to write even though it is overall a worse experience.
Then the best most can do is two views (mobile and desktop) which adapt to some various widths in an obvious way, using loads of whitespace as a solution.