When exporting a folder with a range of subnets in it, it exports each subnet individually. So when you want to import it back in, you can only to do 1 subnet at a time. Plus the folder hierarchy is not maintained, nor is the description of the folder and subnets.
Import and export could be a good way of backing up IPAM but as you can only import 1 subnet at a time and all hierarchy information and descriptions are not kept, it's not very useful.
Currently the only way to restore a 'missing' IPAM folder or subnet, is to do a full restore of the database which means all current data in all Solarwinds products is lost between the current time and time/date of the backed up database.