Going slightly off-topic here, but Flanking II cavalry have 60% retreat odds. That means that even if they have practically no chance of victory they will survive more often than not, hopefully damaging the defender. This is why cavalry stacks need no siege units. Combine with Airships for even better results. Oh, and if you've done your job right your cavalry will mostly face longbows, and the FII promotion neutralises their free strikes.
And then there's Stables, giving you a second promotion without having to run Theocracy or Vassalage. Yep, cavalry rule.
On topic, an Impi rush is something I have to try. I can see them beating conventional chariots quite easily, given that an Ikhanda gets them the Cover promotion. That's assuming the enemy don't get metal. Even if they do, their mobility improves your chances of cutting the metals before too many axes arrive.