Well, if reformatted and updated/reinstalled then it possibly overwrote some of the deleted sectors - rendering some files unrecoverable.

As well as the potential overwriting. File carvers have trouble recovering fragmented files. They might produce corrupted files, or nothing depending upon the details of the file format.

The update that hung (and was forced-to-completion by allowing a reformat) was a major revision. (It might have been IPhone OS 1->2, even... I don't recall) So not only did the reformat touch sectors all over, but the package of standard stuff grew, and I even think Apple's default partitioning scheme may have been adjusted as part of the update.