As a lazy man, the only reason I hand out kingdom titles is to deal with peasant revolts. The peasants rise up against their count in some backwater part of my empire that I only conquered to make my borders pretty? Well, there are two liege levels between me and him (his duke, the duke's king) with the resources to deal with it.
I also love this perk. I hate having to send armies to distant lands just to crush a few rebels. It also means I can't declare war and react to situations as fast as I otherwise could when I have troops raised.
It's risky. Kings will take themselves far more seriously than dukes and counts. They'll be less likely to submit to your demands and more likely to want independence.
On the other hand, it makes things a hell of a lot easier to manage. Instead of having 60 vassals, you only have five or six.
In my experience everyone submits to my demands equally. The only chance that comes to mind that is not the same is assassination chance. However, as far as I know, other empires may have more issues as Byzantine and Rome gain some nice bonuses making it easier to control an Empire.
Actually, in a game I'm playing as Byzantium now, I destroy kingdom titles upon acquiring them, so I can keep a tighter grip on my vassals and so they won't be yearning to usurp my title
I've never had anyone try to usurp my title. In my experience, it was always pretty easy to keep them under control. Once in a while, one or two kings might get upset and want to try something stupid, but everyone else liked me so much that nothing ever happened. I think something people forget about is a gift of gold. For around 100 gold you can get something like a 60 bonus to their opinion, which is normally enough to simply get them to disband a faction in my experience. This normally allows me to go until either they, or my ruler, dies and allows me to start fresh. In the case of a new ruler it also works well, and the bonus lasts until your ruler is much more established.