Office 365 usage reports permissions:

       Limitation: Who can see Office 365 Reports? You must be a global administrator in Office 365 or an Exchange, SharePoint, or Skype for Business       administrator to see Office 365 reports.

Assigning admin roles:

Assigning admin roles with PowerShell:

View your access and usage reports:

There is different ways to achieve BUs delegation:
o   Split to multiple tenants (other impacts)
o   Use a packaged third party offer (for example
It should also be possible to use group management provided by Azure AD (Azure AD Premium subscription required), but that need to be confirmed in term of feasibility and supportability:
Azure AD Delegated Group Management: Feature walk through


Please note that Azure AD Premium currently offers (in Preview)  a solution about delegation: