They can be among the harder parts of a toolkit (though it also depends on the toolkit's scope - can't really compare something like Qt that has a very large scope going even outside GUIs with something like imgui, which only provides the most barebones GUI functionality) yes, but i think the "hardness" exists not in how hard they are to have that functionality but in most people not even thinking about these things.
Accessibility and internationalization are really hard though, right?
I'm finally doing something about the accessibility issue: https://github.com/AccessKit/accesskit Still have a long way to go though.