The migration that reported success
The worst outcome is not a failed migration. It is a failed migration that told the customer it worked.
A failed migration costs an afternoon. A migration that writes COMPLETED over a broken site costs trust, because the customer finds out before you do, and they find out from their own visitors.
Every step that can fail must be able to fail the job. A step whose result is appended to a log and never read is not a step, it is decoration. A handoff to a human is an honest state and must never be dressed as completion.
If you can only afford one rule: nothing writes COMPLETED that has not been proven serving on the far side.