There is a significant deficiency that makes Activity Sub-grid unusable in many situations.
The only link that the Activity sub-grid can use is the Regarding, which means the follwoing activities will not be included in the Activity sub-grid, yet they will be included in the Activities folder (on the Account/Contact entity):
Any Activiteis where the Regarding is a Opportunity/Quote etc. (no rollup of activiteis from related entiteis). Appointments where the Account/Contact is a participant and is not specified in the Regarding. Any emails where the Account/Contact is a To, From, CC and the regarding is not explicitly set to the Account/Contact. ALL emails created by the email rotuer if you choose to integrated all inbound emails for existing Accounts/Contacts/Leads.
This leaves so many exceptions it defeats the purpose of displaying Activities in the sub-grid in most situations. It would be great if Activity subgrid would use the same filter that the Activiteis folder has.