Trying to figure this out since yesterday morning. First I had to defeat the weird super dark shadow artifacts with smooth terrains which actually seems to come from non-clamped displacements (huh). Soft minimum and maximums to the scalar feeding sandwaves disp cured it more than subdivision.
Now, though I'm having strange effects along the horizon of a dune. The dunes terminator, and sand wave peaks have weird warped scanlines. I can't get rid of them. What are they? Absolutely nothing in the scalar info that should be creating these. Is this just lighting and angle creating artifacts? If you look in the foreground, it's not present, but middleground, there starts to be weird dark shadow lines on the peaks, and than they get closer together into the background.
This like my 20th attempt at trying to avoid this, and still can't get rid of them. I don't notice them as much in PT, but it's also slow as molasses with sand highlight and speckles (though looks 10x better).
Preview is zoomed in with TGs render view to see the issue better.