On a Sponge server, user tried importing an export file that was exported with the --without-users flag and it throws an NPE:
https://pastebin.com/v2TT4W3Y

Haven't tested with Bukkit/etc but I'm guessing it may be the same since this points to /common/ . I found it in the code and told a user to use it, and the import failed. Another solution is to simply export all and import all, and then bulkupdate the users away. However, the flag works on export so it should probably work on import too. It's due to the importer looking for "users" and finding no JSON obj. for it, as far as I can tell

Just thought I should let you know! I'm also going to work on a PR to fix this today, might as well give it a shot.