Often it's a great opportunity for housekeeping - retire all those old Adobe Reader 8.0 packages and suchlike you don't use anymore.
I've found on migrations that lots of things can mess it up and lots of the imported data, even when successful, isn't much use.
For example, if the Owner of a NS6 item has been deleted from AD then that will stop te data being migrated. For some types of data it will just stop the data with the missing owner, other types will stop everything from the data with the missing owner onwards.
Imported legacy Pacakges can't have their expiry set, so if you want to keep them indefinitely on clients you can't.
If you Import (rather than migrate) MSI packages the Detection Rule is created for you.