I've backed up and restored ~14-15TB[0] with their B2 product. IIRC sync each way took three or four days, not really that bad. I used rclone, though, not any official client. And that's B2, not their "unlimited" backup service.

[0] My home file server, migrating a four-disk mirrored-pairs ZFS array to RAID5 including replacing the smaller pair of disks with ones matching the larger pair, so the old ZFS filesystem had to be totally destroyed in the process and I needed somewhere to put the data for the like 15 minutes the logical disk wouldn't exist in any form. The alternative would have been to build an entire new four-disk array, doubling the disk cost of the project and requiring some kind of second host-machine. This approach saved me $400 or more, probably wouldn't have attempted it otherwise, cost would have been too high. Ended up costing somewhere in the tens of dollars as I recall.