> This is because OpenGL is at its essence a 2D API. The vertex shader accepts something as input and it produces 2D vertices in device coordinates (-1 to 1) as output.

And the same can be said for every other GPU API. That's what rasterization is. You take a bunch of input and put out a 2D texture. It's necessarily 2D because screens are 2D.

Luckily, first class support for matrices, 4D vectors, mipmaps, lighting (well, at least insofar as lighting is math), etc make it well suited for creating the illusion of 3D.

I get paid to work with OpenGL -- and, somewhat uniquely, 2D OpenGL. I'm confident in saying that while OpenGL might suffice for a 2D game engine and yes, might allow you to skip over a little bit of the math, it is terrible as a 2D UI engine.

GPU APIs are first and foremost for rendering 3D scenes. You can certainly use them for 2D but how well suited they are to that is dramatically dependent on what the use case is.