Boudica's chariots only did anything to the Romans because they were caught by surprise while marching in column. The chariots could punch through thin, barely defended lines and get out the other side, or throw spears and turn away without retaliation.
When the same chariots faced a prepared legion massed for battle, they were totally ineffectual. The Romans could stand behind their solid shield wall and the horses would shy away from the line, making a breakthough impossible and any thrown spears did little, if anything to the Roman infantry.
In that decisive battle, the Britons lost a reported 80,000 men to the Roman's 400. Thats a terrible loss by any standards.