It's not necessarily hard but it is a ton of work and requires a lot of low level knowledge of the underlying window system. You need to have a very good understanding of the layer beneath you (xlib, Wayland, win32) and how fonts are rendered on the various systems you want to support. There's also various tradeoffs you need to make. Do you go with custom drawn widgets or use native ones? Native offers a shortcut if you have a greater understanding of the lower system API. Custom drawn requires possibly less knowledge of the various widget API but more knowledge of the underlying window drawing API. Getting basic buttons and labels drawn is not hard. The harder parts are menus, text entry, scroll bars and scrolling. None of the current toolkits were cross platform from initial release. Almost all started with Xlib and then years later were ported to Windows. Some still haven't made the port to OSX (except to rely on x11). It's a lot of work and maybe that's what makes it hard.
