Changing the shadow bias removes the shadow acnea altogether. The documentation writers of babylonjs have done a really good job about writing about the code and explaining the concepts of 3d behind it. For others who might be facing the issue of shadow artifacting or acnea you can take a look at the troubleshooting section of this page.
https://doc.babylonjs.com/babylon101/shadows#troubleshooting
You can also take a look at this opengl article that is linked in that page that talks about shadows and shadowmaps and removing shadow acnea
http://www.opengl-tutorial.org/intermediate-tutorials/tutorial-16-shadow-mapping/#shadow-acne