After much experimentation I have come to the following conclusions:
If you have missing geometry from extreme displacements in your scene you should try the following, in this order:
- Increase the Max Bucket size in Render/Advanced/Bucket Controls. Default 256. Increase to 512 or higher.
- Try switching off "Allow auto reduction" in same Render Bucket control.
- Increase Displacement Tolerance in the Planet node. Default is 1. Try 2 or larger and reduce, until problem reappears.
- values over 3 indicate you have more serious problems.
- values over 4 will make your renders too slow to be useable. Find another solution.
In some special cases - you can get a good solution by just rendering a cropped region around the failure point, and by lowering Render Quality. But these last two will only help in weirdly catastrophic situations involving unusual node connections such as multiple compute terrains. Generally these last two will not help you but they do indicate where the problem lies.
Following steps 1-3 and trying them one at a time will result (if successful) in keeping your render times as low as possible.
You will need more memory as the Bucket size increases - so this may place a limitiation on you.
Increasing the Displacement Tolerance will directly affect render times by a large factor. Try it last.
It is noticeable that the effect is amplified if the displacement projects into the camera viewport. So looking down on a mountain might render well, but looking up at the mountain will exacerbate the error and parts of it might disappear...