The problem with "every string is Unicode" is if you want to represent things that look like unicode but aren't really guaranteed to be unicode. This includes filenames on Windows (WTF-18 aka arbitrary WCHAR sequences) and Linux (arbitrary byte sequences) that are interpreted as UTF-16 / UTF-8 for display purposes but limiting yourself to valid UTF-16 / UTF-8 means that you cannot represent all paths that you might come across.