When creating a Limited Administrator, you can assign access for OUs in different categories, but you have to assign them to each OU and sub OU - they are not inherited. For example, if I wanted the helpdesk staff to only be able to run reports (but not make policy change, etc), I would have to grant Read Only rights to each OU and sub OU in the Manage groups/Group Rights.
We have 2 main OUs that are synced from AD with about 100+ sub OUs....giving those permissions is very tedious. Oh, and you have to do that for each user account that you want - in our case 3 helpdesk staff members. You can't copy an Admin account or their permissions.
It would be much easier and logical if the access was set on the parent OU and then it would be inherited by the child OUs. So if you have Departments as an top OU with Finance, HR, Development, etc as sub OUs you could just assign the access at Departments.