You can still import MH12 3D elements into MH13 documents, but MH12 does not handle complicated 3D objects very well - and you end up with bits missing. Also you don't get the advertised benefits of the new MH13 3D objects.
In the diagrams below there are 2 Sleighs set at 90deg angle. They should interact as the Moho12 example. But in Moho13 either one or the other is totally on-top/in-front depending on layer ordering.
