Granted, they are lethal to cavalry, as anyone who has played as the Britons will tell you
However, they do not really kill infantry; rather they just knock them down and disperse them. The best strategy here is to charge your cavalry in straight afterwards and rout them. The main issue I have with chariots is that they are bogged down too easily. When that happens, they are destroyed. Every single time. While elephants are not as lethal, they can be moved out of tricky situations and saved from being skewered so much.
As for flankers, I have found that pikemen are only useful in formation. While they can obviously be taken out of formation for running, I find it too laborious to micromanage them. But that's just me
