I mean this is the only type of shader that you can assign to the Material style, and this type of shader should not be used for filters.
I mean that they post-process a subtree of VisualElement. The subtree is rendered before its filter(s) run. But it’s fundamentally post-processing because it applies to a quad that encompasses whatever was rendered in the quad. It’s not like it applies to the actual UI mesh.
Yes absolutely, be careful to use the provided UVs. Using an unlit shader graph shader should mostly work but some nodes might not work. Also normal/tangent won’t make much sense and you must not move the vertex positions. It’s the kind of stuff that we would like to make clearer by having a dedicated shader graph target for filter shaders.