[armchair_expert]

First of all, this is an immediate rendering library which is a very inefficient way of rendering the UI for any application that is not a videogame. You are rendering the UI 60 times a second, regardless of whether something changed or not. Most UI frameworks only update when something changes.

Secondly, there are things in GUI frameworks that most people don't think about, such as accessibility. Windows has a builtin accessibility API where apps can declare the objects being shown on the screen, so that screen readers can interact with the application. I'm pretty sure this guys framework doesn't have that.