Whilst the ustar format may be more 'modern', it doesn't support adding files over 8GB in size to tar files.
So at time of writing, if you need to work with tar files that contain files over 8GB, you can't use PharData.
Archives based on the tar file format follow the more modern USTAR file format. The design of the tar file header makes them more efficient to access than the zip file format, and almost as efficient as the phar file format. File names are limited to 255 bytes, including full path within the phar archive. There is no limit on the number of files within a tar-based phar archive. These archives can fully compressed in gzip or bzip2 format and still be executed by the Phar extension.