Thunderbird only supports moving from one mailbox to another, it doesn't support syncing whole namespaces. It also doesn't support syncing with namespace rewriting or syncing flags, timestamps or access rights. It does not allow using different authorization and authentication and is unsuitable for large syncs as you can't resume anything or have a progress bar. It is also pretty slow, because it does one mail at a time and is just not designed for this.

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.

I wonder if it includes workarounds for Google IMAP bugs etc. Some years ago I migrated over 10 years worth of emails from Google to Fastmail and I ran into a lot of issues with Google's buggy IMAP implementation. Things like a random 1 in every 1000 email failing to delete so I would have random emails still remaning. I found out it was a known issue which Google pretty much decided they WONTFIX. I later found Fastmail had its own migration tool that (I presume) has workarounds for various bugs in other providers IMAP implentation making it easier than using e.g. Thunderbird to do it. If I could go back in time and use a tool like this that has a way to work around Google's bugs that would have saved a lot of my time.

I moved from gmail to fastmail a few years ago

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

You could use Thunderbird, but it’s slow for large mailboxes and multiple accounts. Being cloud-based, Migrate Wizard moves email faster, tracks progress, retries on errors, and supports incremental syncs - all without running a client locally.

does it matter how long it takes if you're going to do it once?

last month I moved 20 years of email

I started offlineimap before I went to bed and it finished before I woke up

> does it matter how long it takes if you're going to do it once?

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.

Thunderbird stops the whole process on first failure. You won't even know from the log file which email is at fault.