Death & Taxes. A mod good for some things (More formable nations, increasing variation, expanded gameplay that basically makes it more interesting to people used to the game, spices up the game for oldtimers, somehow the AI blobs more properly, etc...) and bad for some others (It has severe speed issues due to bad processing and a number of bad decisions, eg. it lasts 100 years after EU3's timeframe while not really doing anything about the gameplay after 1820 iirc.) like every other mod out there.
I think modders and Paradox alike have a tendency to make really bad decisions in at least some aspects of their game. Currently I enjoy
More Provinces Mod (Which reshapes the world properly and beautifully, moreso than D&T, changes the core game less, but dumps Greenland as a colonizable area - which pisses my Danish imperialism off - and ahistorically introduces pseudonationalism from game start onwards) but there's also
Magna Mundi which is infamous for its sky high complexity but ******** interface and slow programming and a number of total conversions.
When my friend played as Aragon in a multiplayer game, Castille wouldn't leave its neighbours alone, including him. It's very rare that Iberia is unified by anyone, but there's war in that area most of the time due to benefitable conquest and unification missions telling the AI to go amock. Don't be surprised when they attack you. Often, Castille is more powerful in Europe, but if you handle your expansion correctly with a powerful navy (great naval tradition, equal tech and a powerful fleet) you will own them with colonial superiority and win wars through attrition and conquests of their colonies rather than the brute might they often possess. Oh, and Iberian lands themselves aren't particularly wealthy save a minor few. If you want to deal with them permanently at some point, see to it that you align yourself with France or be opportunistic and devastate them while their pants are down, usually fighting France taking their Centres of Trade or something when making peace.