Why do large mailbox migrations to M365 fail?

What people tried

Every workaround mentioned in the threads below. We haven’t tested any of them — and nobody here is claiming they worked.

1. 1
   Temporarily using a higher-tier license with a native 100GB cap to perform the migration, then switching the license back.
2. 2
   Turn off automapping via PowerShell and manually add the mailbox to download the local file.
3. 3
   Run SCANPST to check for and fix database errors.
4. 4
   Create a blank PST file and manually copy folders or use drag-and-drop while disconnected from the internet.
5. 5
   Use third-party paid conversion or mailstore software like CodeTwo or MailStore.

In their words

Unedited, most upvoted first, each linked to the thread it came from.