Right now the Autodesk Civil 3D reader in FME 2018 doesn't support Assembly and Subassembly entities:
AutoCAD Reader: Entity type of 'AeccDbAssembly' found, but not supported. Skipping entity
AutoCAD Reader: Entity type of 'AeccDbSubassembly' found, but not supported. Skipping entity
Working with infra-BIM projects we often encounter these entities in Civil 3D data.

