Brittel and inflexible, perhaps, but not ineffective, and hardly stupid. Towards the end of the Zhou dynasty the blast furnace was invented, which could melt iron ore, which could be casted into weapons, armor, or anything else you wanted. While not as durable as worked iron, arms and tools could be manufactured in substantially greater numbers. Blacksmiths in Europe and the Middle east could never outfit an army as enormous as Qin's.