Even distant relatives have the potential to claim your throne. Be very careful if you give out county titles to your dynasty members. try to find relatively young characters of your religion and culture who have the content trait. when your empire has become a giant sprawling blob and you have a couple hundred vassals, you may want to start creating duchies and handing them out if you have gotten crown authority to absolute (free revocations), in order to better rally your entire empire's troops during war. This may cause you to hold too many duchies.

Also, don't create duchies in regions that you plan on being a part of your demesne. Do not usurp titles if you don't have to, for the same reasons as above. Additionally, if you own any provinces within a duchy, the duke or duchess of the region may be upset by this fact. While they give a temporary prestige bonus and casus bellis for quicker expansion, in the long run they cause more instability within your kingdom, as vassals will attempt to acquire these titles via rebellion or some other means. Don't create additional duchies or kingdoms.