The migration of my in-house SBS2011 went well while following your migration guide. The guide is extremely thorough and easy to follow.
I do have a question about the Mobile Devices that appear in the Windows Server Essentials Dashboard. There are only two devices that appear in the list, but there should be at least one more (see the attached screenshot). The two that appear are my wife's iPad and iPhone. My iPhone does not appear in the list and I don't know how to make it appear. I have tried deleting the email account from my iPhone and re-adding it, but that did not work.
My devices do appear in the O365 Exchange Admin Centre (see attached screenshot).
Do you have any idea how to get my iPhone to appear there?
Strange that one phone does not appear. Maybe you should wait for some days and see if it appears. If it does not maybe if you recreate the email profile does the trick.