Take in account that this is just preliminary observation, but I had quite a laugh.
First turn on Hard as Tokugawa, i have a nice juicy Oda army on my doorstep (in my territory), and a convenient free metsuke. I send him to the oda army and bribe 3 units away, then I proceed to wipe out the rest with my troops, then withdraw, planning to siege Owari (oda's capital) the next turn together with the troops I bribed.
I end the turn, and Mr. Oda, instead of taking a sensible defensive position, since he has way less troops than I do, he just grabs the few guys he had left in his castle and runs into my territory (to achieve what I don't know, considering that his army is now more or less half as mine, and if he tries to besiege me, i'll obviously crush him). Completely suicidal.
But the laughable part still has to come. I end the turn and one of the neighboring clans of Oda (with which they were already at war, making their move even more stupid) , the Saito Clan, just happily walks into the completely undefended Owari, obliterates the Oda clan, and the "invasion" army in my territory just poofs.
Come on, seriously? The ones described by the campaign as my arch enemies, and one of the most powerful clans of Japanese history obliterated on the *first* turn because they did an absolutely stupid suicide attack on their neighbor, leaving their own capital with *zero* defense to be taken by the other neighbor? Is this the extremely smart CAI boasted by CA?
Just for for testing sake, assuming it might have been just a temporary brainfart of the AI, i restart the campaign, once, twice, and again. It happens every single time.
I even try on normal to see if the AI is less stupidly suicidal, but no avail. They still move on me and the Saito walk into their capital unopposed. Try it out if you don't believe me, testing it takes more or less five minutes per attempt.
I'm not surprised that people are lamenting several clans being destroyed in the first turn. It's not because they fight epic battles beyond the fog of war. It's because they make suicide attacks on their neighbors, leaving their capital completely undefended to be taken by someone else, instead using simple, elementary logic and building their forces and consolidating their position and THEN starting to attack other clans.
Seriously, the CAI may be challenging (probably because they overly raised it's tendency to blitz), but as for being "smart", no, I don't think so, at least from what I've seen so far. In previous games generals and rulers were suicidal on the battle map. In Shogun 2 they're suicidal on the battle map AND on the campaign map. lol.
CA sure managed to teach me something about "The art of war", despite I read it multiple times. I didn't know there was a secret chapter that started with "Leave your home undefended, and then go suicide in foreign lands".
Good job