Nearly every failed migration we are asked to rescue went wrong in the same place: the cutover was treated as a single event. Mail, files and identity were moved together on one evening, and when one part misbehaved there was no way back.
We stage it instead. Identity and multi-factor authentication go first, because everything else depends on them. Mailboxes migrate in batches with coexistence in place, so a delayed batch inconvenienced nobody. File structure is designed before anything is copied — lifting a messy server share into SharePoint simply relocates the mess.
Each stage has a rollback checkpoint and a defined owner. We also plan the human side: short onboarding sessions before cutover and floor support in the first week, when the questions are small but constant.
Done this way, staff notice new sign-in prompts and little else. That is the goal — a migration nobody has a story about.