Interesting historical debate. My point of view:

1. It is possible to train horses in a manner that they run against and crush in a firm obstacle like a dense body of men with or without spears, pikes and shields. It would be obviously not very good for the horses health and might be their one and only battle but it is possible.

2. It must have been a fairly seldom event at any ancient, medieval or modern battle that there were together enough horses well trained in the described manner to achieve a contact or break against a firm infantry formation.

My opinion is based on the fact that in war horses fade away very quickly even without actual fighting (desease, starvation, accidents) and at a far higher percentage than men. Maybe some of the supposed breaks were achieved by knights with such trained horses but imho it happened not so often to take it as a rule (unfortunately I could only list incidents where medieval cavalry was unable to break massive formations but I hope for enlightenment).

3. The reports of frankish knights breaking through their byzantine and islamic enemies should not be taken too seriously. My explanation for this strange hint, if it isn't mere exaggeration, is that it is meant for cavalry against cavalry. In this case heavier cavalry has a big advantage. Maybe another example for this effect: in some battles of the 30-years-war the imperial Kürisser/cuirassiers in heavy armour are said to go through lighter swedish cavalry "like a knife through butter". That was not the case against infantry.

4. For me the mongols greatest advantage was not the bow or horse or mobility but the very good organisation and discipline which equalled those of modern european armies. And it's not a miracle that steppe people performed so pitiable against modern european troops of equal or better discipline from the 16th century on.

5. Cavalry in MTW reflects knightly combat not in every instance. It behaves more as modern cavalry fighting in seperate bodies and being able to charge, summon, withdraw and recharge several times. Medieval cavalry was not always able to do this in a similar manner. So I would appreciate if cavalry wouldn't be as strong as in RTW where sometimes balance is ridiculous.

6. Even if cavalry wasn't able to break massive formations from 350 BC till 1890 AD frequently battles were won by cavalry and that will be also the case in M2TW, I presume.

7. sry for my english