At first glance a customer portal appears to be a frontend topic. In practice, however, its usefulness stands or falls with the same questions as for any process-oriented enterprise solution: who is allowed to see what, which data are authoritative, and which processes must remain auditable?
It becomes particularly critical when desktop applications, back-office processes, services and web access rely on the same data source. In that case it is not sufficient to merely build an attractive interface. Roles, authorizations, API boundaries and versioning logic must interoperate.
If this connection is implemented cleanly, no parallel secondary system emerges; instead a consistent access for customers, employees and administrators is established. This very aspect later determines maintainability, security and extensibility.