I've only had issues with world scales that are too small or too large. If the effect is working but you're experiencing problems, try scaling the entire scene.
A large grid size will always increase projection errors, since the static cubemap position can end up differing too much from the camera capture position. Try to keep it around two or three character steps in size.
If you want to debug disocclusion, uncomment line 307 in cuby_marching.gdshader. This will highlight all disocclusion areas in magenta (the "Fallback to Cubemap" option must be enabled).
If you encounter other issues, ensure that you're using Forward+ and Godot 4.5.1 or 4.6.3.
If you still run into issues, feel free to reach out — I’ll be here to help until everything is working properly.