The hardest part of Exchange to Office 365 migration that clients often underestimate is not the data copy itself, but the surrounding complexity of identity, coexistence, and change management. Mailbox moves are relatively straightforward compared to handling hybrid configurations, DNS cutover, authentication methods, and maintaining seamless email flow during transition. Many organizations also overlook hidden dependencies such as legacy PST files, shared mailboxes, calendar free/busy consistency, and third-party integrations tied to on-prem Exchange. Permissions mapping across Exchange, SharePoint, and OneDrive can introduce unexpected errors if not carefully planned. Network bandwidth, throttling limits, and migration windows also impact performance more than expected. Finally, user disruption, training, and post-migration support often determine success more than technical execution. A successful migration requires careful planning, pilot testing, and strong governance rather than just a migration tool or script alone in most enterprise cases.