What are the risks of heavy customization (modification) to the SAP core system?

Direct modifications to core SAP objects create a form of technical debt that compounds specifically at upgrade time, but also carries ongoing risks even between upgrades.

• Upgrade friction — modified objects can conflict with SAP's own delivered changes, requiring manual reconciliation ("SPDD/SPAU" adjustments in classical terms) on every upgrade.
• Support complications — SAP support may be limited or require additional diagnosis effort for issues occurring in or near modified objects.
• Knowledge concentration risk — deep custom modifications often become tribal knowledge tied to specific long-tenured developers, creating a bus-factor risk as those individuals leave.
• Blocked cloud migration path — heavy core modification is often the single biggest technical blocker preventing a move to S/4HANA Cloud, where core modification isn't even possible.

These risks compound over years — the longer an organization defers addressing them, the larger and more entrenched the remediation effort becomes by the time a cloud move or major upgrade forces the issue.