I don’t know about Fusion drives, but APFS is fully functional in HDDs, internal or external. However, Time Machine will not accept APFS-formatted drives for backup. If you try to add a hard drive (APSF format), it will offer to erase it as HFS+ before adding it to the Time Machine drives list.
I even converted a number of USB thumb drives to APSF with no problem (except that they probably cannot be read on older systems). When I tried to create a High Sierra installer in one using the createinstallmedia command line, it also gave an error with APSF and reformatted the thumb drive before I could use it.