I'm trying to set up some greeble panels and I keep getting these undulating faces when I use a displacement map. I am using a pretty high res 6000x6000 PNG to generate the displacement but I can't figure out why the faces aren't smooth. The subdivision level should be high enough to accommodate the details (its on level 4 with 10,000 polys to start) but it doesn't for some reason. This happens when I use the node render based displacement through cycles as well as the evee method with using subd and displacement modifier. Here are some screen shots to demonstrate what I am talking about.
I have a few possibly helpful pointers:
Foremost, I have noticed that Blender doesn't always play nicely with .png files (especially with normals and displacements). Try a .tiff, or other type. I had the same issue of 'undulating surfaces', surfaces that should be smooth, but appear bumpy for no apparent reason.
Second, I notice thin, horizontal rectangles in your second image. Check that your mesh geometry is composed of squares, not rectangles. Otherwise you'll experience issues when displacing or otherwise deforming the mesh.
Lastly, double check that your UV's are unwrapped properly, same reason as above. If the UV is stretched in one direction and not another, it will distort the effect. Not sure if this is the problem here but thought I'd suggest in case.
One more thing: you might want to try using adaptive subdivision in this case. Andrew Price does a good job explaining what it does and how to use it in this video.
I hope one of these helps!