What if you've got 83 vassals and 30 of them don't like you, or 18 vassals and 6 of them don't like you? Its easier to manage in a new succession or if you're going to change the laws.
Less vassals are easier to manage. The downside is the risk that they pose. A king-level vassal has enough power to invade other realms. This is great, until he comes to pose a threat because he makes up a significant portion of your power base.
You also lose the ability to manage the dukes that fall under the king, allowing them to grow, marry and extort their way to power as well. The main risk with King-level vassals is that they or their Dukes will start marrying outside of their De Jure borders, and you might end up with a King blobbing within your borders and a lot of resentment and minor wars between vassals.
I tend to make a lot of money and prestige by micromanaging my Dukes, so overall I prefer a lot of one-duchy one-county dukes to a few kings, but that is just personal preference.