Well, as the Khergit Fanatic that I am, I wholeheartedly disagree with you. To the contrary, while they ARE weaker in sieges in multiple cases, because of the Horse Archers, they actually have a significant amount and decently strong archer defense during sieges. Plus they, have a lot of troops to serve as soldiers in towns and castles. Now, on the field, they are slightly underpowered. However, I believe that they actually do a decent job at attacking. Without my help, the Khergits have managed to take multiple towns from Vaegirs, Sarranids, and Swadians.
Well, on the open field, they steamroll infantry. Unless they get stuck, which then they die, they do impressively well on the field.
Oh well, that's my opinion!