Ok, I have seen a multi-company rule there. I suppose there might be a problem when:

1) A hierarchical structure defined for a model with a single reference to the parent.. (e.g. parent_id )

2) The multi-company access rules are applied (the user can see the records according to the hierarchical structure of the companies)

I suppose that complex validations would be needed to restrict the operations over 1) in such a way that parent chain is not broken by the visibility  restrictions of 2). Specially if there are customizations for access rules .

Maybe that is the reason ? Would be then an implementation based on parent_lef and parent_right a possible solution ?