What was the biggest probelm you and your members had to overcome?
The only problem for some members was getting to grips with the fact that they really don't need Tapatalk any more. (Although I think you can have it on xenforo). I hate TT and couldn't wait to ditch it.
For me:
Converting CMS articles to resources (there may be an importer for that though but I wasn't aware there might be at the time)
What I did though was copy and paste the articles, but there was only a handful tor two. Those by other members I used a change author addon. Then made sure I had redirects in .htaccess
Dealing with PMs.
No problem if you don't want to limit the number, but I needed an addon that allows a conversation quota, and the only one I now of is paid for and is quite expensive if all you need it for is the inbox quota. If you can persuade your members to export and delete their PMs (in and out) before the migration it could be useful. If you do want a conversation quota, you then need another addon for exporting.
Media embeds
Not a big problem this, but you need to search and replace youtube embeds, there's a
really good addon which will do this and you only need to run it once.
If you don't mine sharing? How big was your forum?
Much smaller than yours, about 4000 members 20,000 posts.