Triangle Facing Direction
Rhino typically displays all surfaces as two-sided, regardless of the direction of their surface normals. However, the Unreal Engine automatically culls triangles that face away from the camera. This may cause some geometry in your scene to disappear when viewed from certain angles.
For example, the image below shows a wheel well cover that was modeled with its surface normals facing inward. After the scene is imported into Unreal, the sides at first appear to be missing. In this case, flipping the normal direction of the object makes the sides appear as they should.