From the fsck man pages, only ext2, ext3, ext4, and reiserfs support it. I hope that `btrfs-convert` deals with it correctly, but you must not ever run `btrfs balance` or delete the backup (it won't waste space if it was a fresh FS). And if further bad blocks develop you'll have to redo this all from scratch.
From experience I can tell you that most usb sticks fail while reading back data by returning bad data, not an error or timeout, and the defective sectors aren't always the same (internal remap to wear leveling might be the cause of that).
Advice: Do md-raid0 on half of them, md-raid0 on the other half, and btrfs raid1 on the two md devices. If you want to exceed 25MB/s, use different USB controllers, not different ports on the same hub.
Bad blocks don't count as allocated unless they are newly discovered - data should be moved asap if it's still recoverable. An allocated block with nothing in it is an error.
I would not recommend this setup, Any real drive would be better. I just hated wasting drive bays for the non-critical boot/system partition. In my defense despite burning through many cheap usb drives I have not lost the boot yet, however I do keep a new spare drive taped to the unit.
It lasted a couple evenings while I bumped into apparently every Linux USB driver/chipset issue possible trying to drive a couple hundred drives via cheapo AliExpress USB hubs.
It was fun times, but even more useless than I originally thought it would be. It was amusing to get >1GBps for a minute or two across 256 crappy thumb drives though!