umm...

[Tollheit]

I can rout one unit of Hoplitai with one unit of cavalry by frontal charges, and that is not how cavalry should be used.

[Vuk]

The Roman Empire (and Republic) DID use slingers. The reason that they did not dominate the battle are threefold.
1. They could not be concentrated easily like archers, because they needed a lot of room to shoot (I have done it myself, so I know :P)
2. If the enemy hard large shields they could get up to enemy slingers taking very few losses.
3. To be a good slinger took a lifetime of experience (which is why I am such a lousy one :P).

As to the cavalry comments, I am not saying they should be able to roll pikemen (like the guy who did the tutorial tried), but they should be a lot more mortal (ei, take a heck of a lot more damage), and do a LOT more charge damage. (and no, I am not saying that they should launch enemies like Vanilla had) And to avoid confusion, I am talking about heavy charging cavalry, not horse archers or something. Every unit should be more mortal, ranged units overall need to do more damage, infantry has to be a little slower, shields have to count for a bit more, and shock troops need more charge damage. That is my (un)proffesional opinion.