The best apps I've used have implementations for every OS and UI separately. Usually, everyone uses the easier route, but it will only be good enough, not the best. But again, now your app works only on Windows.
Even better if the library code is properly written, not only you can have multiple GUI frontends, you can make the CLI folks equally happy, and most of the code remains portable.
Naturally nowadays this is too much to ask for, so many ship the Chrome Application Platform instead.
