Prefer proven foundations for standard work
Accounting, purchasing, inventory, access control, audit trails, and master data are expensive to rebuild well. An ERP usually provides a stronger foundation when the process is common and integration matters.
Build when the workflow is distinctive
Custom software is justified when a workflow creates strategic advantage, serves a unique user experience, or cannot be supported without distorting a standard system. A hybrid approach often keeps the ERP as system of record and adds focused applications around it.