Thomas Brand, in a good piece on why Fusion Drive is obsolete:
Fusion Drive, no one needs your fragile combination of storage technologies anymore. If Apple had really believed in you, they would have made you the default storage system on a Mac, any Mac. Instead you were neglected to a build-to-order-option. Even the top-of-the-line 27 inch iMac never shipped with a Fusion Drive standard. Fusion Drive, we will always remember you for the fast plentiful storage you promised. A great idea whose time came too late.
True story: until reading Thomas’s article, I had totally forgotten Fusion Drive even existed.