alt Hacker News1 v 1, the average nomad could kill the average conscripted peasant who was physically weaker and less experienced. An iron vs a flint spear isn't going to make nearly as much of a difference. Superior numbers for the creation of a professional class of fighters that conquered the weaker nomads, but the steppe nomads remained superior in combat. The Roman, Chinese, Persian, and Muslim empires were only able to keep them at bay by turning them against each other. When they united, they were completely unstoppable.
With the Romans, the situation was the opposite. Their success was mostly based on the idea that conscripted peasants will eventually beat elite warriors. You just had to equip and train the conscripts instead of wasting the resources on the elites.
Because every man was expected to fight, the Romans had an effectively endless supply of experienced and well equipped soldiers. A society depending on a warrior class might win once or twice. But the Romans would still inflict some casualties. They would learn and adapt, and come back with another army next year. Sooner or later, the warrior class would be depleted, and Rome would prevail.
Eventually the Roman Republic grew large enough and successful enough to switch to a professional army. Not because it was better, but because the population was too large. There were not enough enemies to fight to make conscription useful.
Steppe nomads were far from unstoppable. They had occasional success in conquest, but their societies were set up to fail. The legitimacy of their leaders was based on personal relationships between the elites. When the leader of a large empire died, it was always unlikely that all leaders of note would support the successor. Most of the time, the empire would fracture into effectively independent polities. Sometimes there would be a figurehead leader on the top, but he would rarely have any real power over other leaders.