With this mindset, is there a reason why the progress bar for tape backups cannot be accurately measured?
For example I have a tape backup going here. It is writing the latest full of all of my backups to tape. All but one task, with a rather large VM, is completed.

It has been sitting at 99% for several days even though it has been writing the entire time. Prior to this VM starting, progress was fairly consistent (perhaps not perfectly linear, but close enough to think the progress bar was semi accurate). It seems when it gets near the end it stops being accurate and gets near Backup-Exec levels of progress bar reliability.
As a programmer I am not sure I understand how Veeam could not know what the amount of data to write would be,since as far as I am aware, it is just writing the backup files to tape, and the files can determined in length beforehand. Sure there are things that could affect the *time remaining* (if there was one), but the amount of data to write, the amount of uncompressed written, and the amount compressed written could be determined.