Well I don't war much, but I think the major concerns are that they get Commando, which makes them able to harass cities and loose units both extremely effectively, because they can move so far into a territory on the turn you declare war, and can move in and out of range of nonmounted units. This makes defending pretty hard, especially for the AI and it's ridicilous road placement. (Though by the industrial era, all tile improvements except farms benefit from roads/railroads, the AI really dislikes having an outdated road t) High withdrawal rates further increase this is, as the mounted units are more likely to survive. (Because they usually retreat a step backwards, and so don't tend to die much, and because you can use them to damage fortified units without losing them). Then you add in Great Generals, which can increase your odds immensely (A fully levelled one is.... 0-2 first strikes, +50% withdrawal, +50% strength, +healing etcet.) and the AI doesn't use these at all. All this means that in an equal war, the AI is in pretty dire straits against a human opponent.