When to create sub-organisations or set up as own entities

I have a client (public sector) who has dozens of semi-independent sub-organisations. I’m aware that C/A/D can include these under 1 umbrella organisation, so I’m wondering how to best set them up on the platform- 1 account each, or 1 organisation with ‘business units’. With a limited number of accounts on the system, I’m inclined to go for 1 with sub-units. I’d welcome any thoughts on pros and cons for either approach