The Linux "copy problem"

Reflinking does, inevitably, reduce the fragmentation-resistance of the filesystem if you use it a lot and tend to keep the post-reflinked files for longer than the pre-reflinked ones. (That's the whole point, after all: the new blocks are necessarily going to be fragmented wrt the old ones, even if this is reduced by writing more than strictly needed in the CoW phase.)

Ideally, a background preen phase would rewrite such things after a while so they are non-fragmented again. I guess one could call xfs_fsr from cron to do that... on an SSD this matters not at all, of course, and not very much if there's a caching layer like LVM's or bcache in the way either. But it's a real concern on unintermediated spinning rust.