Found a small bug, might have been mentioned somewhere along the line but didn't see it in forum search.
When digging the top of a cliff, it seems the cardinal direction the cliff is facing has an impact as to whether or not you can dig it. Best way to describe this one is to show it, so I made a quick video: https://www.youtube.com/watch?v=YyUaOOZ6r1w
In the video I'm positioning myself so that the survey will attempt to dig the terrain vertex along the cliff face. Notice that the first attempts (wherein the cliff is facing south) the character just moves to the edge and doesn't attempt to dig. On the other side, facing west, the digging works fine. The results are the same with regular Adventure->Dig method.
A workaround for this is dropping dirt at the bottom of the cliff and letting it decay into the ground, raising the ground and removing the cliff, making the area diggable. (thanks sabinati for that pro-tip!)
Also an unrelated minor bug/annoyance with cliffs, (which I'm sure the devs are aware of): if you get close enough to a cliff to go into the "cliff mode" (where moving parallel to the cliff sidles back and forth, and you can jump down the cliff), your character no longer responds to any other command but clicking to move. The character won't respond to build, lift, dig, clicking survey, etc. commands while in this state.