I'm playing my first campaign on H/H specifically for this reason, and also so I can get a handle on the game mechanics first.

But even on H battle settings, I've had a few unexpected tough losses (particularily during seiges), and also, some frustrating battles where I managed to maneuver the AI into an outnumbered battle on the campaign map, thinking I'd be able to destroy their units with minimal losses, but only to incur big losses myself.

Even in spite of the "passive AI" bug, which only seems to happen to me on occaison, I think the battle AI in this game is much better than in RTW.