I wouldn't worry about it too much. If you run the mount command when connected to the box you'll see the flash filesystems are also mounted with the synchronous option (from my ALIX setup below):
/dev/ufs/pfsense0 on / (ufs, local, noatime, synchronous)
devfs on /dev (devfs, local)
/dev/md0 on /tmp (ufs, local)
/dev/md1 on /var (ufs, local)
/dev/ufs/cf on /cf (ufs, local, noatime, synchronous)
devfs on /var/dhcpd/dev (devfs, local)
Synchronous means the system will sync all writes and not return from a command until the write has been completed. Even if a power failure were to happen with the filesystem mounted read-write it shouldn't cause any lost data as commands don't return until the write is verified as on-disk.