Here is a simple setup where the cube has an Uber emission shader and the Environment Light has been turned off.
Sample file attached.
When rendered with the Uber Shader to 250 samples and as seen in the preview above the render has lots of black square artefacts.
Here is the Uber Shader final render to 250 samples:
However... if you swap the Uber Shader for the Blender Emission node instead the render doesn't have the black squares.
Here is the Emission Shader final render to 250 samples: