Everything has already been said here, but just to confirm:
- Old displacement + autobump currently results in incorrect shading. Using new displacement with autobump is preferred, or old displacement without autobump.
- New displacement can sometimes cause artifacts if the original mesh is not subdivided enough (e.g. flat plane with no subdivisions at all), if it has long, thin polygons, or if it has chamfered edges.
- We are looking into all of these issues, and hopefully in V6 we will have improved 2.5D displacement, which would make the old displacement obsolete.