The hardest is to make multiline text input and rich text support. None of the smaller, home-brown GUIs gets this right. You basically have to write a complete editor that supports all languages, including Japanese, Chinese, Arabic, etc. Even native operating system text input widgets have been plagued with problems. Generally, there are tons of edge cases and special input considerations to consider in a GUI, e.g. DPI scaling, accessibility, right-to-left languages and unusual unicode glyph handling (composed glyphs), scrolling behavior & mouse handling, platform-dependent layout issues (HI guidelines), escape characters in file paths and unusual file systems, and so on and so forth.

It's quite possible to make a good GUI, just lots of mundane small issues to deal with, and it's overall a massive undertaking. It requires at least an extremely good knowledge of how different languages deal with text and input it.