The problem
ERP and MES don't reconcile themselves
Material master, bill-of-material and batch data have to stay consistent between ERP and MES for production planning and traceability. Consolidating plants or moving to SAP S/4HANA is where gaps start breaking reports.
Process manufacturers carry an extra layer: batch genealogy that has to survive every change, and test systems for ERP/MES integration that need real-looking data without real exposure.