I've been playing a campaign as Rome (Julii). Had 3 children, 2 of which died before coming of age. The third came of age, and I had him raise an army.
Prior to this, I completed my first civil war without issue. That left me with 3 political factions–my own, and two competitive factions. When my next civil war began, I noticed that my only son decided to create his own political faction to replace the faction that had rebelled. It seems like there must always be at least 2 competitive factions when government is set to Republic, but selecting family members to replace rebelling factions–thereby ensuring that there are always at least two other political factions–seems to be an awful, immersion-breaking mechanic.
My faction controls nearly 80% of the senate, controls 6 armies (compared to 4 of the 2nd faction and 1 of my sons'), and had nearly crushed the rebels in a single turn. Why the hell would my only child–my heir–leave the family faction to form his own when it puts him at a massive disadvantage and instantly puts his life in danger? He's 15 years old and only has influence over six senators! Six! Versus my 400 senators. And he has like 9 gravitas. The dude can't even get married. So the decision to leave the family makes no sense. And now when he rebels (and he will at some point due to political mechanics), I'll need to kill him and end my bloodline after a single generation. Awesome.
The point of having family trees is to grow them. I can't do that if my children are arbitrarily selected to replace factions every time a civil war occurs. Use the "Other Nobles" or just generate a new general and faction.