Well, Counts recieve no Tech Point progression, so their holdings will stop being upgraded after a while. Aside from that, the more vassals you have the more there are to join plots against you. Likewise, if you can keep one or two Dukes out of factions by keeping them loyal with gifts and your chancellor that will seriously blunt factions.
On the other hand, creating Dukes will reduce your maximum income and levies, though over time that's offset by tech progression.
Creating a vassal Merchant Republic with a member of your Dynasty is a good idea though, the Doge will pay a lot in taxes, his levy will include lots of ships and because Doges are elected they tend to make highly skilled Council Members.
As to having a Duchy of your own - that ensures you'll hold onto a portion of your power even if you lose the Kingdom to a faction. It also increases the rate at which you generate prestige, which can be useful for a number of things, including impressing your vassals.
Never create Duke who have a Demesne more than 50% the size of yours, though.
On the other hand, creating Dukes will reduce your maximum income and levies, though over time that's offset by tech progression.
Creating a vassal Merchant Republic with a member of your Dynasty is a good idea though, the Doge will pay a lot in taxes, his levy will include lots of ships and because Doges are elected they tend to make highly skilled Council Members.
As to having a Duchy of your own - that ensures you'll hold onto a portion of your power even if you lose the Kingdom to a faction. It also increases the rate at which you generate prestige, which can be useful for a number of things, including impressing your vassals.
Never create Duke who have a Demesne more than 50% the size of yours, though.
