Most custom objects in HubSpot are leftovers from a Salesforce migration that nobody bothered to clean up.
We use a three-question test for every custom object:
- Is the relationship many-to-many with a default object? If not, it's probably a property.
- Does the entity have a real lifecycle independent of contact/company/deal? If not, it's probably an association.
- Will more than one team need to report on it monthly? If not, it doesn't earn its place.
If the answer to any of those is "no" — kill it.