The AI in the game is great in some ways, managing wars, especially if they have plenty of units, deciding to attack or not to attack a particular stack and so on.
It is good in other ways, pretty decent at managing their economy and research, good players will beat it, but that's gaming.
Then it is also abysmally poor in other ways. I like to play for a while as a nation, then save and load up other nations from that save to see how the AI is really doing. How about EVERY country having 5 magistrates sitting there while they have 1 or two star great men? Now, playing as Biz you are pretty lucky if you start with a 3 star, or even have 3 choices to start, but over the next 20 years or so, you can hire yourself a full crew of 4-5 stars. The AI SUCKS at that. It obviously does not know how to manage magistrates.
In one game I remember the Horde did not even have 5 merchants in its own CoT, and had all 5 merchants sitting along with 5 of everything else (only 2 spies though).
Bottom line, the reason why you see people saying that Navarra is easy to win with is because, other than some completely random luck on Aragon's starting mission, you should have no problem beating the AI if you are familiar with all points of the game. How about joining Castille in a war on Morocco and grabbing a province to produce Charge Cavalry. You will be 50 years ahead of Aragons cavalry then. The AI does not even use the best units available to it most of the time, despite it only takes about 3 months for the morale to tick back up to 100.
So, what's ridiculous is blaming the game for running into too much trouble when it is very handicapped in a lot of ways.