Flattening of a surface allows the strain of a screw to relax, which in turn correlates with a loss in tension and thus preload.
Therefore, embedment can lead directly to loosening of a fastener joint and subsequent fatigue failure.
Only embedment that occurs after installation can cause a loss of preload, and values of up to 0.0005 inches can be seen at each surface mate, as reported by SAE.
Exceptionally hard and smooth surfaces will have less susceptibility to the mechanism that causes embedment.
However, the relaxation in strain is less significant to the longer grip length and the loss in preload will be minimized.