I have a question please, I tested a very simple scatter asset HDA both in Unity and UE4.
- to change the instanced mesh in UE4 there is a GUI.
- but Unity does not show any GUI to allow change the mesh.
Is this a bug or am I doing something wrong??

Yes, that is a current limitation of the plugin. I added a bug to track this. In the mean time, you can add an object merge and expose the object path as parameter.

Thanks seelan, I will do as you say……