So I'm having the same old migration issues. With a twist.
I thought the migration had all gone swimmingly until I wanted to add one of my new media files. No sign of it in the Upfront "Add Media interface" or media gallery. However all the media was in the wp-content/uploads/2016/12 folder. Yet the site was pulling images from localhost. Embarrassingly I didn't realise this for a while.
When I was restoring my full site snapshot it kept getting caught on wp_posts so I left it out as nothing had change with the blog. When I restored without it, the snapshot restore completed but with some very strange exceptions (although maybe my understanding of wp_posts is a little basic).
So please WPMU are you doing ANYTHING to improve the migration of linked images and replacement of localhost with live URL?