I think the 3 absolute demons that will eventually seek to destroy you in any homebrewn GUI framework will eventually be:

- Rendering and dealing with text in all of its extreme permutation. Styling. Localization. Character sets. Etc.

- Modern accessibility standards. There are countless annoying yet de facto UX patterns that are just nightmarish to check off the list from scratch. Inputs/forms/etc. Even worse than accessibility, because you can decide to forgo that completely if you really wanted. The sheer volume of UX use cases and edge cases involved in even seemingly basic input components is also an exercise in rote dread. You've got input validation, error states, just so many ergonomics to deal with.

- Fluid layouts and scaffolding. This is where modern HTML/CSS finally starts to shine because making flexible layouts is actually quite painless. But it's a motherfucker to roll your own approach.