Kingdoms suffer far more stability issues than a Duchy of the same size.

[xcrissxcrossx]

I'm not sure if this was reported officially, but it seems as if dukes are far more aggressive to their lieges than counts. If one starts as a duke, it is far more safe to not form any kingdoms when they go on a conquering spree. Even if one plays as a king and takes over another kingdom whole, it works much better not to form the kingdom. You do not get the prestige bonus, but it seems as if you get a relationship hit from all your dukes because they seek the king title that you hold.

 

[K-vald]

I would consider this WAD to be honest. It's a question of planning, if you want to have a kingdom then it will come with some negative effects, but if you do not want to have a kingdom then you don't have to.