I'm sure there are many issues with GUIs. My main one is that lots of things can't be (easily) expressed in the API of common languages. For example you set the colour of something to red? Well, that thing renders in some specific way that calls a callback which in some cases will override the background, so setting the background colour will not do anything. You can't express that in objects, there's really no failure here beyond "you didn't consider every possible layer and combination of features". Yet, whenever I try to create some GUI, I run into an issue like that every few minutes.