I was able to complete a lot of Yahoo Groups transfers this week. There were still issues of some missing files and photos, and the invite system throws up errors every once in awhile. But it was manageable.
Unfortunately, this morning, it appears there's a sporadic issue with Y! Groups' archives. Some of the time when I request a specific message, Yahoo will return an internal error message. Which means we end up with only partial archives. I've halted all transfers for now until this hopefully clears up (there are currently around 30 groups in the queue to be transferred).
(The group transfer process is unfortunately not fully automated. I have to respond to each invite, because of the captcha. And I need to oversee the crawling of the groups and the checks for space.)