UC are cowards who can't take down rebels or criminals, but are happy to put the boot down on civilians who won't fight back. The good news, their sheer size and political stagnancy also make it hard to collapse, so they are stable, for all their faults.
HN are perfectly capable of destroying their enemies, and aren't afraid of anyone. However, their sheer stupidity means they shun stuff like medicine and technology, and have a political system that collapses the moment the current leader dies. Because training an heir creates a rival, not training sets it up for infighting.
In short, the world of Kenshi is screwed no matter what faction wins. This is why you, the player character with starvation-induced brain damage and a seemingly randomized moral compass, should take over.