Currently FlatCubic doesn't seem to support any types of structure whatsoever, not even the types of structure that CustomCubic can support. As vanilla flat does support structures, support for this should probably be added, possibly with further customizability than vanilla.
The following structures are supported:
Vilages
Mineshafts
Strongholds
Temples
Dungeons
Decoration features
Desert pyramids (weird vanilla implementation of this, FC can do better)
Currently FlatCubic doesn't seem to support any types of structure whatsoever, not even the types of structure that CustomCubic can support. As vanilla flat does support structures, support for this should probably be added, possibly with further customizability than vanilla.
The following structures are supported: