Normally when a king assigns a fief plenty of lords get jealous. Default range for change of relations with a faction leader is between -5 to 2, which means that if a faction wins a war and a king will assign all the land they conquered, they all will start hating on him. Let's say they won about 10 centers, and on average every lord will get -2 relation for every center assigned to somebody else, then there is a very high chance that some will leave, with castles and cities which they hold and destroy the faction this way.
I played games both ways and I definitely prefer to have the relation change in the range of -2 to 2. Much more stable, though I ended up with almost no lords in my own kingdom, as everybody is so happy with their king.