Well I started a hard campaign the other day.

So far the AI's battle strategies seem about the same, although I've been more cautious because of a need for troop conservation.

As you've said though the AI seems much more agressive and unfriendly on the campaign map. And holy crap these stupid peasants won't stay down! Four successive rebellions in the same province, and all because the province was 75% Christian! Jeez! Why isn't there a "cull unruly peasants" option for the Metsuke?

That said I've also found that I see the Wokou pirates a lot more in Hard than normal. Although I captured every one of the five ships they sent against me.