My biggest issue with flavor packs is their focus on specific dates, while other DLC, even events packs, are timeless. Northern lords focuses only on 867, completely ignoring other dates. Most content of Persia also focuses on first date, while in Iberia situation is a little better. Other DLC designed with future ages is mind - cultures get more and more traditions through time in Royal Court; player open new accolades and tournament activities along with different clothes and armor in Tours and Tournaments; Roads to Power gives us the Fourth Crusade and clothes progression. In most cases this progression through time is not that big, but at least it exists.
There is only one way to fix flavor packs - add content to all start dates. Why we have only Vikings, while completely ignoring Normans? Why there is no constant Norman adventures in 1066 Mediterranean, who terrorizes Africa coast and Roman Empire? Why rulers of Scandinavia wear clothes of tribal era in medieval era? Why are they stuck in Scandinavian succession, while every other ruler around have primogeniture?
With fate of Persia we get Seljuk invasion, but didn't get aggressive Turk adventurers. Conquest of Anatolia and formation of Rum was one of the biggest disasters in roman history, and yet, we didn't see this in 1066. Seljuks just conquer Armenia and explode. And what about Zoroastrian faith? Why can rulers embrace it only in first start date? Zoroastrian is not dead even nowadays.
In Iberia situation is better, but its struggle is still very annoying and problematic. Persian struggle can end without player or AI interaction, while this struggle will continue up to the EU4 times. There is still one of the most annoying conditions of ending this struggle in aggressive way - we need to spread our culture and religion. Religion part is easy, but culture is annoying even after vassal directives introduction. Also, struggle may end in creation of tiny empires, which is absurd. Another big issue - is when outsiders come to Iberia - for them this region simply doesn't offer anything. The quintessence of this problem is Almohads - game consider them outsiders and doesn't recognize as a part of struggle.
As for event packs... Friends and Foes, in my opinion, was a mistake. Instead, this DLC should focus on house feuds entirely. House feuds are one of rare factors that can make game harder even for experienced players. Even if player character survived in a feud with giant house - his children may be not that lucky. But this feuds feel abstract and disconnected, sometimes even confusing, because I can't normally see progress of them. CK3 itself may be described as a game about house feuds and instead of them DLC focuses on birthdays and tavern drunks. House feuds literally ruined Tibet Empire and yet, there is no possibility of starting feud on religion basis in this DLC.
The other two event packs is much better, because they give us working and useful mechanics. Hostages, adoption, university visit, better education control, three traveler activities and whole new lifestyle. Their events are repetitive, but some of them are really interesting (such as hostage events, where you can adopt them and make part of your dynasty).