Are you sure the yields are slim? 32 science and 32 culture and 32 faith from killing a single unit? Even if you discount faith, that's 64 science/culture per unit, which doesn't sound bad?
As Kwami pointed out, early those yields are ok, as they can equal an entire turn worth of science and culture.
But my example of 32 science and culture was referencing a Cuirassier kill.
Whenever I fight AI Cuirassiers, I'm already at that point in the game where I'm around the Industrial or later era myself, meaning that I usually have at least 200 science and culture per turn, often a lot more.
At that point, a unit kill is worth
at best one tenth of a turn worth of research (or far less even), which is for all intents and purposes, nothing.
Hence this ability scales awfully with eras, being decent'ish in the Ancient and early Classical eras, and more or less worthless later on.
For a domination type leader, that's a pretty damn weak selling point.
I should also point out that I play almost exclusively multiplayer against the AI and never have SS on.
Hence, I get no early envoy for being the first to meet an AI, and no super early governor.
Meaning the levy gimmick in this case is somewhat iffy.
And if I can't properly utilize this ability in the ancient/early classical era, Varangian Harald is for all intents and purposes the worst leader in the game right now.