Its true, that layered TIFF is not just PSD embedded in TIFF but more like a TIFF container implementation of the PSD Format. Technically layered TIFF is practically as native to Photoshop as PSD - there is only one thing missing: Duotone.
The same problems of compatibility with PSD account exactly the same to layered TIFF: One point is crucial - there always seems to be a mismatch between _import_ and _export_:
Import means how much of Photoshops features get recognized in PSD/TIFF and applied to their Affinity counterparts. Think of Grouped layers, layer modes, adjustment layers, smart objects (!) and so on. What about smart objects which use third party Smart filters?
Most people think PSD/TIF-Support is a symmetric thing - but this is absolutely not the case. Most non-Adobe programs can IMPORT more Photoshop-Features than they can export. This may sound illogical, but exporting from a Program like Affinity to PSD means mapping Affinity-native Features to Photoshops counterparts (which may not exist or not be really accessible). While on import the developers can extend their own Features to cope for Photoshop-Behaviours - this is not possible in the export direction.
The route to support layered TIFF besides of PSD in Affinity opens up collaborating with tools of different vendors on some file to some degree (to the degree how good the TIFF/PSD export is - which is limited).
The route to embed .aphoto into a TIFF would be perfect for collaboration with RAW workflow software like Capture One or Lightroom. It would be the PERFECT answer to all those questions about a DAM-Module. Those programs only need the merged down version of the image to export or to apply their own non-destructive adjustments on. Editing such a TIFF would just replace the internal .aphoto with a changed one and update the merged down layer. All nondestructive adjustments in the workflow software will then apply to this new merged down image. Since there is a .aphoto in the file, Affinity Photo can actually export any feature that is possible within this suite into it. It would also be possible to open the TIFF in Affinity Designer to add some vector stuff. The downside of this approach is, that any other program would just see the one merged layer and editing it with such a program may destroy the internal .aphoto.
I personally really like the "embed .aphoto" approach - this would be the one approach that would allow me to drop my CC subscription. As long as "layered TIFFs" would only offer exporting limited features of Affinity Photo - this would be well... limited.
So in an ideal world there would be both options supported. Or even all of those:
1) Flattened TIFF (as now)
2) Layered TIFF, compatibility maximized (as good as possible to Photoshop layered TIFF)
3) Layered TIFF, features maximized (contains .aphoto and a flattened Layer)
4) Layered TIFF complete (combination of 2 and 3 - no flattened Layer)
To me only 2 and 3 are really relevant and if I had to choose only one it would be 3.