I took a look at a few VMs that are still on btrfs - Most of them have the 'ssd' flag set when looking at /etc/mtab, even going back to SLES 12, despite not having it specified in /etc/fstab anywhere. The ones that go read-only whenever a btrfs balance runs and need the root filesystem to be restored from backup each time are not showing the ssd flag in /etc/fstab. The backing storage is "all-flash arrays", some of which have NVRAM as the first stage before data written to them is deduplicated, compressed and written to SSD. A similar issue was also seen on a SLES 11 physical system with HDD using hardware RAID, so while lack of the ssd flag might be inducing extra wear on the all-flash arrays, the corruption induced by btrfs balance appears to be independent of that option. That SLES 11 system was aging, however, so it's possible that its hdds were beginning to have timeouts, though.