File names are a different problem because Windows and Unix treat them differently: Unix treats them as bytes and Windows treats them as Unicode. So there is no single data model that will work for any language.
This means that there's more overhead on Windows, but it's much better to normalize what the application programmer sees across POSIX and NT while still roundtripping all paths for both than to make the code unit size difference the application programmer's problem like the C++ file system API does.
Seems like an apt acronym for Windows... :-)
On a more serious note, Python seems to have done something fairly similar with the pathlib standard library module.