On the import of the SMF 2.0.15 forum, the importer does import subforums, but imports them as main forums, not as subforum under the correct main forum.
Thank you for reporting this issue, it has now been resolved. We are aiming to include any changes that have been made in a future XFI release (1.4.1).
Change log:
Correctly import nested forums from SMF with the correct parent forum.