Hi Antoine
To answer your last question, yes, there is a reason of importing an Ifc instead of a datasmith format exported by Revit or Archicad, or other. The reason is that most of the files we import into Unreal are in IFC format. Of course it works with a datasmith format, and Unreal correctly import all materials of my multiple layer wall.
Have a look in this capture : this is a wall with multiple layers and Datasmith plugin has correctly imported the materials :
But, in this exemple bellow, I’ve imported the ifc format with datasmith plugin and look : the imported wall got only one material furthemore the result is bad :
So, I’ve created a Python script to parse my IFC file to get correct informations and my script give met that information about my wall :
My problem is, how to modify the geometry of element to attribute the good materials into the wall imported by the datasmith plugin?
If I edit the StaticMeshActor, I do not know how to have a correct wall geometry like it was on a datasmith file format?
Does your ifc parser is not correct?
Whould my solution be to rewrite my own importer?
Thx Antoine. Hop you’ll undrestand my english… That’s funny because both we are french i think… 
Have a look here : when I import my ifc file format into Revit, he correctly import my wll with 4 layers and good materials of each layer of my wall.
Also with Archicad, th ifc wall is correctly imported…
Here in Archicad :
