Most migration horror stories share a root cause: content moved before the destination was governed. Before the first wave, stand up Entra ID conditional access, Intune device enrollment, and Defender baselines in the new tenant — then let mailboxes and files land inside controls that are already enforced.
During the move, map permissions instead of flattening them. Staged waves with rollback checkpoints mean a bad batch is an inconvenience, not an incident, and same-day verification catches broken shares while the old system is still available for comparison.
After cutover, close the loop for your auditors and insurers: document who has privileged access, retire the legacy servers formally, and capture the evidence — conditional access reports, device compliance, data loss prevention policies — while the project knowledge is fresh. That single folder often pays for the migration at the next insurance renewal.