For me it's a disappointment. I think one of the biggest problems is that the vassal system is entirely borked. Vassal opinion basically doesn't matter at all, because a vassal that is your best friend and confidant will give you no more troops than a vassal that hates your guts. Also, factions almost never fire, and if they get close, you can just execute a few criminals and all your problems will go away. Moreover, because vassals give such a small amount of troops (and only poor quality troops) kingdoms are ridiculously weak in CK3.
In CK2, the amount of troops a ruler got was partially determined on how much their vassals liked or hated them. A king with only 1 or 2 counties in his demesne could still be an effective ruler if his vassals liked him. And if some foreign power invaded, they'd receive a large opinion bonus so that the realm would actually rally against the invaders. Thus, even a kingdom stuck in gavelkind could still have a reasonable amount of power, depending on the ruler and situation. (The reverse was also true; a vassal who hated you might not give you any troops).
In CK3, a king with one or two counties is probably weaker than almost all of his dukes, and even some of his vassals' vassals. And if a Viking invades, intent on taking all the territory in the kingdom, his vassals will not lift a finger to help save their territory or send so much as one extra soldier to the battlefields. Who cares that they are about to lose their territory as their villages are burned and their daughters forced to be concubines? Their vassal contract says they will contribute 25% of their levy, and that's that. Since all Catholic kingdoms are stuck in gavelkind and vassals are worthless, it leaves the Catholic kingdoms pathetically weak. It also means that the player, who manages succession so that they don't lose all their titles, will very quickly become overwhelmingly powerful without even really trying.
The lack of start dates is also frustrating. I have never liked playing the 1066 start, since Europe is pretty much dominated by the HRE and Byzantines, but in CK3 the only other start date available is 867. But 867 in CK3 kind of sucks. The vikings are overpowered and, since there's no conversion mechanic, they'll never convert. So unless you conquer all of Scandinavia, you're stuck with annoying raiders for the entire game. And thanks to the new technology system, you're also stuck with a small demesne, a crappy succession system, and, depending on your culture, may not have access to your cultural troops for the next 200-300+ years.