In an unmodded version of RTW, chariots can be very effective. The trick is to leave them out of combat until the enemy is weakened and their units are wavering, then send the chariots through the enemy units to start the rout. Often a chain rout will ensue. I had a brief campaign as the Britons and found my general's chariots were unstoppable at times, completely destroying the enemies' morale and sending them flying at the same time. The only thing is chariots need considerably more 'babysitting' than cavalry because they're so vulnerable to unprepared attacks. They can also be incredibly weak to the degree they just dishearten me completely. I once send a unit of British light chariots to attack a unit of Screaming women who had their backs to the chariots. I watched in disbelief as half my chariots fell to bits when they collided with the rear-ends of the German women. I don't know what those women are packing in their behinds, but it was too much for my chariots. After that I lost faith in them. Cavalry are much more reliable, easier to manoeuvre and generally more effective in the role you give them. So whilst chariots have their uses, I prefer cavalry by a long shot.