Answer
Only the data needed to support real sales and service decisions should move, but that set is usually larger than teams expect. Account status, open orders, invoices, shipment history, service notes, and key contact information often matter more than a narrow contact list.
Buyers should define the system of record for each data type and then decide whether sync needs to be real time, near real time, or scheduled. The best design is the one that supports decisions without creating constant reconciliation work.