So fer the only advantage is reduced number of micromanagement with your vassals, when you have to deal with 1 kings instead of dozens of dukes.
The downsides are more serious. With latest patch, kings tend to always join some kind of faction to make your life more miserable. No matter what pinion they may have on you, even with 100 they will join independance, even if they are of your ciulture, religion and so on, or lower crown authority.
So is there any advantage in making kings? Duke you can manage somehow but with kings they may get huge bonus of manpower to the faction to allow it to rebel.
The downsides are more serious. With latest patch, kings tend to always join some kind of faction to make your life more miserable. No matter what pinion they may have on you, even with 100 they will join independance, even if they are of your ciulture, religion and so on, or lower crown authority.
So is there any advantage in making kings? Duke you can manage somehow but with kings they may get huge bonus of manpower to the faction to allow it to rebel.