It does fail at both countering artillery and destroying building for a 2 pop unit and a high cost… At 36 siege atk every 6 sec and 2 pop, its super useless. Consider a mortar at 500 dmg every 6 sec for 4 pop. Pop wise you get 2(arrow knight for 4 pop) * 2(number of attack in 6 sec since they have rate of fire of 3) * 36 = 144. Pop wise it is impossibly bad. Ressource wise it is still really bad, they are less cost effective than a mortar at destroying building.