The problem with cavalry isn't that they are too powrfull but they should be much more expensive and/or harder to get (because of of lack of trained horses or something).
Cavalry even without much armor was a killing force even against well armoured hastai and principles. Romans wasn't using it that much not because they were a crap but because they PREFER heavy inflantary and were traineg in that. Even roman legates and tribunes wanted to win their glory on foot. And they weren't that good in commanding cavalry because there wasn't any cavalry tradition or tactics...

Look what did spanish cavalry under Hannibal's command...