Javelin cavalry vs. Horse Archers in an interesting question. Javelins pack more of a punch per shot, but the number of shots is a lot fewer (they do less damage overall). Horse archers cause less damage per shot, but they have more shots to use.

The key issue with Javelin Cavalry is range - exceedingly low and, considering that they are lightly armoured, this makes them very vulnerable to longer ranged enemy units - often losing most of their men before they can even fire their first shot. Horse Archers are equal range with most archers, removing the time it takes to close in.

Cataphract Archers, I must confess, are brilliant units - the ranged power of a horse archer combined with a large proportion of the melee and armoured strength of a cataphract is pretty much unstoppable. Other unit's missiles are deflected and, unlike horse archers, they are still very useful when their own ammunition has run out.

Their only flaw is speed which is exactly the same as a standard cataphract - very slow.