There is also a huge advantage with dynasty members as vassals. Sure, they don't get the opinion bonus, but you could pick any single one you'd like to be your successor. Same dynasty means no game over. Sons turned out poorly, but third cousin Konrad seems like he'd make a good Kaiser? Go for it.
Exactly. As the King of Naples in my game I can still elect my cousin who is King of Sweden, my Aunt who is Queen of Jerusalem, and brother who is king of Croatia. I don't elect them but I could very easily.
If you are able to setup your dynasty members in a good separate realm it will never ever be game over. When I originally set my brother (Italian) up a hundred years ago in Sweden. I had to constantly bail him out of trouble for over 50 years until his Swedish heir finally took over which calmed all of his vassals. But now he always sends between 5k and 8k of troops when I call him into battle.
Elective is very useful when you have created separate dynasty branches in other realms.