Hello,
If you have an FBX file I would import it directly in Unreal. Just normal import, Datasmith import will not work anyway unless file was generated from VRED or Deltagen.
If the model you import contains tessellated meshes (FBX is tessellated data), datasmith or normal UE import will just import as is. No simplification.
If you import a CAD file (.3dm) from rhino, then it will depends which options you setup in UE:
If you select import as Nurbs, UE will retessellate available Nurbs geometry based on the settings you select in the import window.
If you select to import Rhino mesh it will import available tessellated data as is, and discard tessellation settings form the import dialog.
If you are exporting a uDatasmith file from Rhino, the exporter will get Rhino to tessellate Nurbs data to triangle as uDatasmith file do not carry parametric data.
In that case the tessellation settings can be changed in Rhino
Please note that Rhino allow you to apply modifiers such as “edge softening”. Those are purely render modifiers and are not stored in the exported geometry.
I hope this covers most of you questions.