So, i was under the impression that assets like blueprints can be used as inputs for hdas so, for example do a simple scatter of unreal blueprints.

Since the change in the type of inputs for newer engine versions, there is no more 'assets' option but the documentation says it could be done with the 'world item' input. However you cannot select an item from the Content browser, only objects from the world.
if you do select a blueprint from the world, the output acts like a static mesh not as a blueprint (i.e: it gathers the static meshes from the blueprint like the merge actors functionality).

i haven't tried an object level HDA but i doubt it makes any difference.

Am i missing something or does anyone have any info on this?

If you want to select assets from your content browser, change the hdas input from 'world input' to 'geometry input'. object level HDA would make no difference, you're right.

Also, to touch on the other point, while there is more you can do within blueprints and houdini api, by default the hda inputs will take the actors of the blueprint results.

Thanks for the help Cody

it seems to work if i set the 'use as reference' option.

if not i get this error in the unreal output log:
LogHoudiniEngine: Error: Hapi failed: Invalid argument given: Invalid id. (D:\Nightly20.0CMake\dev\engine\unreal\5.3\Engine\Plugins\Runtime\HoudiniEngine\Source\HoudiniEngine\Private\HoudiniInputTranslator.cpp:1033)

maybe it helps with something, dunno

im using 20.0.653 and ue5.3