Use the heavy chariots only when the enemy unit is shaken or wavering. Then send in the chariots. The unit routs, the chariots knock them flying, and often sets up a chain reaction. When I played a Briton campaign, my generals destroyed everything, but only when my slingers and light chariots first put the fear into the enemy units.

If you are playing a modded game where units have higher mental stats and possibly the battles are slowed down in general, the poor chariots are weaker and less useful by default as their strength is in causing fear. Up the morale of the enemy and suddenly they have a tougher time. A few points of armour can help boost the chariot's effectiveness, but makes them even more overpowered in auto-resolved battles.