Really interesting, thanks @rolfkleef
To just expand on:
It often is possible to "collapse" a parent and its child activities into a single activity node that has incoming and outgoing transactions.
We've been looking at this with @JohnAdams and DFID, in terms the use case of "fund managers" publishing information about their link in the chain.
In one example the fund manager takes a nested approach - publishing incoming funds into a central activity, and then disbursing through to hierarchy 2 activities that then disburse out to the implementing orgs. This seems to place an overhead on both the publisher and data user, and we're not sure of the benefit
Hence, I pulled this slidedeck together to propose a "flat model", making it less complex to deal with. A trade off could be less detail, but it seems to pay on maintaining utility of the data, versus completeness.
Arguably, the above does not have much direct relation to hierarchies, as we remove them in the flat model. Our core focus is on getting to a point where data can be well-maintained, leading to ease-of-use.