The OPness of Kensai in Shogun was dependant on the unit size.
1 Kensai vs 60 Yari Samurai on Normal = routed YS with significant loss.
2 Kensai vs 120 (or was it 160?) YS on Huge settings = good chance of dead Kensai.

I think a strong but low number unit representing the Hero & his guard/followers should be ok.
I hope to see them emerge from gameplay eg you need an individual soldier to reach a certain fairly high minimum honour rather than just being timed or purchasable.

Problem with a minimum honour level though is that you tend to get significant numbers of a unit reaching fairly high honour at once & you'd need these units to remain rare through a game.