If you are looking to come to mybb, SMF has a converter and then convert to mybb after that. Just an extra step but gets you from Xenforo to Mybb in the end.
That's what I ended up doing. Sadly, you lose passwords, avatars, and private conversations (depending on server configuration). For large forums you need to swap inserts to ignores.
Yeah not the best way, but got it done.
I'll be submitting examples of the database to the guy in the sticky thread asking for some