Very fast unit that can still run away, turn and spin-swing at people chasing. It's very hard to punish them unless you have ranged weapons. The Rhomphaia is very powerful, and thus it's not worth picking the improved armor perk over it since it will be mostly the same amount of hits the unit will receive, from both a Skirmish and Captain's perspective. The sling is really useful, at this point it's a troll weapon, I'd add a small shield, perhaps.
i agree with you too much movement speed gives you all the tools to destroy the enemy infantry. Since most of them are so slow you cant really get punished for it. Its funny how the weakest class by far still got its own hard counter.