I have BTRFS on /, which lives on an SSD and ext4 on an HDD, which is /home. BTRFS can do snapshots, which is very useful in case an update (or my own stupidity) bricks the systems. Meanwhile, /home is filled with junk like cache files, games, etc. which doesn't really make sense to snapshot, but that's, actually, secondary. Spinning rust is slow and BTRFS makes it even worse (at least on my hardware) which, in itself, is enough to avoid using it.