The clans are retarded because of the way their armies are set up. You have a faction that is technologically two generations ahead of most IS armies (1 gen ahead if you count Comstar), troops that are genetically primed for conquest, an economy that puts all their production into war materiel without any noticeable drawback in innovation or productivity and ruled by a polity which is largely united on conquest, if not the reasons behind it.
In canon, it was established that one clan mech was worth three or four IS equivalents? That's gulf war I and Iraqi Freedom level odds. Those odds are pretty much insurmountable for any regular army to overcome if they were fighting symmetrically.
Any faction with these kinds of advantages would be effectively invincible. The only way IS regulars could have fought back is if the Clans were suddenly struck by the retardo disease in the middle of their invasion.
All wrong.
1. The IS was recovering most of the Star League techs, and were, in fact, primed to innovate further on them. The recovery technology was a major factor why the Clan chose to invade, even though they were not quite ready for it yet and were politically divided over it. They saw their advantage eroding.
2. The Clans were not primed for conquest. They were primed for battle, which is a completely different thing. They have no idea how to assimilate conquered hostile, alien territories, they had no idea how to deal with resistance, they had no clue how to hold land. All they knew was attack, attack, attack. The entire Clan culture precludes knowing about this because the culture submitted when conquered. They willingly go over to the other side. That did not happen in the IS and the Clan didn't know how to deal with it, leading to Turtle Bay. Time and again, the Clans were protrayed to fuck up horribly when dealing with conquered territories.
3. The Clans had no economy per se. However, what you said is like saying the USSR had no drawback in productivity trying to keep up with the west. There were major drawbacks. It was explicitly stated that Clan technology other than war stuff were 50 years behind the IS. A captured Clan tech was amazed at the medical and safety and labour technology of the IS. The Clans's advancement were almost wholly in the field of combat, at the expense of
everything else.
4. Again, the Clan did not know what conquest is. Their idea of war is a series of individual one-on-one fights. That is complete bullshit, which the IS taught to them rather harshly. And to say they were united is complete nonsense. One of the major thing throughout the Clan Invasion was the schism between the Wardens and the Crusaders, which led to a War of Refusal that nearly annihilated two of the most powerful Clans at the time.
5. A Clan binary was roughly equivalent of a 3060-tech IS company. At the beginning of the Invasion, they were roughly 2-1 in terms of power parity. It was more F6F Hellcat vs Japanese Zero than Gulf War and Iraqi Freedom.
6. It was invincible only because of surprise and inexperience of IS troops dealing with Clan stuff. The Elementals, for example, were a big shock but once the IS got over it, Elementals became far less of a threat. It is basically a faction with new weapons coming at you. Once you figured out what the new weapons can do, what the limits are and what you can do to deal with it, things became less scary and nasty.
7. And you forgot the most important part of it all: It wasn't a retardo disease. The ilKhan after 3050 was actively working against the Invasion. Ulric Kerensky was a Warden, and his saKhan was Phelan Ward, a IS bondman!
You really have no clue what you are talking about.