Note that I used the proper IMAP terms. A mailbox is what the layman probably calls a folder or directory, a namespace is what the laymen probably would call a mailbox or account.
last month I moved 20 years of email
I started offlineimap before I went to bed and it finished before I woke up
and being the cynical sort of person that I am, I didn't trust the fastmail importer
so I ran it, and also wrote my own implementation using the gmail api (NOT imap), and another using the fastmail jmap api, and reconciled them
100% match (bar the "Muted" folder, which fastmail ignored)
pretty much perfect
I do it regularly for backups and updates. For an update I basically spin up a new server and then sync it. I do not want to wait that long, to see whether an update contained a bug or not.