Unnecessary amount of arrows, unnecessary damage and unnecessary bow readying time. If you don't have any good archers its a high chance that the team with the horse archers will win. Horse archers can ready the bow in a split second and inflict 50 damage if they aim for the chest(which is quite easy to do). Readying speed should be decreased a lot and accuracy should be decreased a bit.
They're useless in the following situations: Island of Deriad, Against any team with equal or more cavalry captains.
They typically trade badly with enemy archers which is to be expected.
They can't hold their own against any other cavalry.
When moving at all their aim is horrible (they cannot retreat and fire accurately at cavalry chasing them, for example).
They are exceptionally good at dealing with shock infantry, which they need to since khuzaits have none.
You're still better off getting the lancers with a bow and arrow instead of this unit, since its the strongest cavalry in melee at the moment while also doubling as a horse archer.