Edit:
Using the latest steam version + CK2Plus-1.32.24
It seems like Heresies are a huge problem in this mod. In my last savegame, virtually all of the muslim states were crippled very early on by it, as a result they were not able to provide an adiquate response to Christian holy wars. The Christians were not immune, though, as both the HRE and the Latin Empire fell to Catharism as well. What seems to happen is that an event shows up that converts the liege to a heathen religion (the one where you either imprison or embrace), and after the liege becomes a heretic, he is extremely effective at converting both the lords under him and the population that, by the time he kicks the bucket, the heresy has a solid foothold in the country.
Also, it seems like the Iberian penninsula has a few too many kings. It is very common to see AI characters with three or even four king titles, and a player character can easily have up to seven (Leon, Castille, Nevarra, Aragon, Andalusia, Portugal, and Galicia), seems very strange. At the very least, make it so that the titular title of Galicia cannot be brought into existance if you already have a king-tier title.
I also have an issue with savegames becoming unreadable around 1370... I am not sure what is causing it.
Characters with the immortal trait are dying of old age.
I can confirm this. Possibly an issue with immortal trait assigned from Character Creator not working properly. There was supposed to be a fix in the last CK2+ patch but it did not work it seems.
PS: Big bug, the MTTH modifiers for positive battle events are wrong, positive traits like Strong, Brave, and Genius give penalties while negaitve traits give bonuses to getting these events.