I've gotten excellent results in the field with a Khergit party, but it needs a few Swadian, Rhodock, or Nord auxiliary foot troops for assaulting castles and towns. Off their horses, they provide almost as deadly arrow spam as Vaegirs, but their melee capability is relatively weak.
Swadians all-but collapsed in my current campaign, down to one castle. Rhodocks and Sarranids have traded a border strip back and forth, but have come out roughly even in the long run, with Rhodocks gaining one Swadian castle on the side, and Sarranids briefly taking one Khergit town until I took it back (and now own it). Khergits have gobbled up a couple of Swadian castles and about a third of the Vaegir land (with 2 towns) so far. Nords have taken a few Swadian castles and towns.
In most campaigns I've played, the Swadians get mauled, being in the middle. Vaegirs rarely do well, but don't usually collapse like the Swadians. The rest are somewhat variable, with the Nords and Sarranids often gaining some ground and Khergits either doing quite well or quite poorly, rarely in between.