Firstly, the Visual Shader GUI in Godot has pretty series issues on my Macbook. Secondly, it's impossible to do any effects with a stencil shader [1].
Most of the useful post-processing effects (bloom, depth-of-field etc) are implemented out of the box, so this also means that there isn't a way (AFAIK) to implement post-processing effects yourself.
Godot also only has simplex noise implemented out of the box, so you need to jump through some hoops to get anything else included (which is super useful in shaders).
[1] also has some images of the way to implement outline shaders in Godot, which is to move the faces of object out by a scaling factor and then render them as unshaded, which leaves some gnarly artifacts at the corners.
Most of the useful post-processing effects (bloom, depth-of-field etc) are implemented out of the box, so this also means that there isn't a way (AFAIK) to implement post-processing effects yourself.
Godot also only has simplex noise implemented out of the box, so you need to jump through some hoops to get anything else included (which is super useful in shaders).
[1] also has some images of the way to implement outline shaders in Godot, which is to move the faces of object out by a scaling factor and then render them as unshaded, which leaves some gnarly artifacts at the corners.
[1] https://github.com/godotengine/godot/issues/23721