Hardware limitations matter more than anything else, and that's why the platform is important. Most mobile devices are still limited to 2048x2048, for example. Only the most recent ones can do 4096x4096. Some older ones can only do 1024x1024. If you create an atlas targeting Stand-alone, you can get a 4096x2048 atlas for example. But if you then switch to a mobile platform, your atlas may be unusable, and trying to change it will always give you an error, even if you are removing assets.