Comment by notpushkin

It often is hard. If you’re using win32 APIs extensively, you’ll have to port your code to Linux counterparts.

There’s also the issue of forward compatibility. Sometimes you just can’t run an old Linux game on a newer distro, while it works fine in Wine. Or it might partially work: for example, I’ve managed to run a Linux build of Heroes of Might and Magic III, but didn’t get any sound, because it relied on some ancient sound API (pre-ALSA; perhaps OSS?). Windows version works great in Wine to this day.

For some game engines though, porting is really easy. There are some piracy groups releasing Linux ports of Unity games (that don’t have an official Linux version) by just replacing the game executable with a compatible one from another game.