Total Incompetent AI.
However, the AI is completely incompetent, specially in the campaign actions. I will put some examples I consider to be critical enough for not understanding the high ratings around the web. In my opinion, this game hasn't passed a decent AI quality testing.
One of the most stupid issues I found (high difficulty): a faction declares me the war, I neutralize its attack and I counter conquering one of their provinces. Having a much larger army, I make a peace treaty with them, but demanding 10k gold, and 6 technologies (including military). They accept my demands. On next turn, they declare the war on me again. No comments needed...
In ground battles, I was able during several battles to fire to enemy troops which didn't take any action. They just stood there while dying, not firing nor trying to run out of range. They took loses until routing away from the battlefield. I would like to specially mention I won a whole battle this way without taking substantial loses.
The moving of the units in the campaign map is foolish too. Playing with Russia (highest difficulty settings), Dagestan declared me the war. They took its whole big army (20 units) to rush one of my farms, while leaving its capital with 2 troops. I conquered it on the next turn with a 4 unit army. As being the only territory they held, the nation was destroyed and all its army disappeared magically from the map...
Naval battles seems to be the strongest point of this game, they are spectacular and fun, and I haven't found any important issues. Indeed, this is what saves this game from not being a clone from other Total War games.
After several hours of gameplay, you find out the most efficient way to beat your enemies is exploiting the AI problems. That's disgusting. I think this game has much more potential left and a lot of things to polish. But like most of the game industry does in these times, the graphics is the only thing it matters...