Increasing morale across the board on VH has the effect of making the AI's troops less likely to rout when you charge. Since the AI's advantage is brute force and yours is finesse it is a slight increase in actual battle difficulty. On medium or easy you can kill limitless numbers of militia troops with two units of heavy cavalry and good timing because they will rout before killing anything. On VH they might kill a few of the cavalry before routing.
All in all I wouldn't say it's much harder, you're just less likely to score immense heroic victories and you'll take a few more casualties overall.

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