I've come to expect this sort of thing, ever since my second job, back in the mid-'80s, when we had to back out an OS upgrade (that was a cold reinstall and reload from backups) of a Stellar GS-1000, because the new version's restore(8) refused to read the leavings from the old version's dump(8).
These days, I've come to really like cpio; while it has many varients, I've had good luck reading GNU cpio's "-Hnewc" format with some vendor cpio programs, and it succeeds in representing the contents of a filesystem sufficiently completely and accurately so I can reload a complete system using cpio, fwhack the boot blocks, and boot the result.
In high-volume high-performance mass backup settings I'd probably hack up something around Amanda, going compression either locally or remotely but in any case staging the whole tape full to a big horking disk array before streaming it out onto the DLT.
Once burnt, forever shy of vendor proprietary format backups.
Besides, my backups might outlive my vendor.
-Bennett