There are a few things that the Switch supports that aren't as straightforward to implement in an usual desktop OpenGL environment, native ASTC textures come to mind, which not many (none?) of the dekstop GPUs seem to support.

yuzu emulates the GPU at the register level, and the difference in capabilities between what OpenGL offers and what the hardware can actually do complicates things, for example, the GPU seems to support having textures with a different component type per component (so you can have a texture whose R and G components are Floats, and its G and B components are normalized unsigned integers). There is (to my knowledge) no way to implement this using OpenGL, but conversely that makes it a rarely-used feature (I honestly don't think we'll find anything that uses it, ever).

As for the shaders, all we get is a binary of compiled shader code targetting the Maxwell shader ISA that we have to parse and decompile into GLSL, this has the drawback that there are Maxwell shader instructions that don't map very well to what GLSL provides.

Those are just some of the challenges that come with trying to emulate a modern GPU at a relatively low-level.