Flash filesystems such as JFFS2, UBIFS, LogFS, etc are designed for direct use of MTDs (memory
technology devices, i.e. devices that expose the fact they are Flash - typically within an
embedded system), not on block devices (i.e. hard drives or consumer Flash drives such as
Compact Flash, USB thumb drives, etc). It may be that BTRFS would work well on the latter,
but it probably wouldn't work on MTDs, as it would need to include an FTL (Flash Translation
Layer) to do wear levelling etc.
Of course, BTRFS ideas and code might be useful in writing an MTD-capable flash filesystem
such as UBIFS/LogFS, but you might as well start from scratch as Flash is a very different
medium to hard disks.