IDK, the Clans and their invasion brings to mind many of the tactical/strategical mistakes of the Germans in WWII.
*Too much focus on quality as opposed to quantity, ease of repair/replacement and cost efficiency(lets compare Soviet weapons/tactics/etc to the German ones and the results)
*Underestimating the determination of their enemies to resist despite fielding less skilled or trained units(again Soviet Union vs. Germany is a great example)
*Engaging a combined enemy force that severely outnumbers your own(though of course it could be argued that this was unavoidable)
Or the Mongols, who ruled the largest (contiguous) empire the world ever saw.
You have a small force that focused almost exclusively not only on mounted combat, but on mounted archery. They almost always were outnumbered. They were born and raised in the saddle, to a much greater extent than the European knights, Arabic nobles, and Chinese that they fought.
I think that the Clan invasion also suffered from one of the failings of the American invasions of Iraq and Afghanistan: The Clans didn't just expect the Inner Sphere governments to rapidly collapse under their combined assault, they also expected to be greeted as liberators, with Spheroid civilians throwing roses on the roads they marched on, now that they were free from the yokes of their feudal lords.