that's the point of the effort after all.
warren as smart as he is was tryna go against the grain of thousands of human evolution. if you train your kids - then most likely they would've values similar to yours. hit or miss but most times a hit.
The other downside of nepotism -- besides the risk that Joe did a shit job and won't admit it or maybe can't evaluate his kid objectively -- is that even if the kid is the bee's knees, a great hire, it's not really fair -- there's almost certainly other candidates who are just as good and trustworthy as Joe's kid but they can't compete with either Joe's kid. It doesn't really matter what sort of references or credentials a candidate can show you if you can serve as a reference for the other candidate. You can't really run a meritocracy like that.
if your biological child is an idiot/wasteman - then you could adopt - the Roman Empire ran on adopted children, with biological children seen as wasteman.
the other way it's done e.g with Japanese businesses - again adopt your son in law and give him your surname.
the thing about giving helm to the sons - is usually you would've had a whole lifetime to evaluate the candidate 20+ years. do mistakes happen yeah, but honestly the risk is overblown.
where that's true or not, that's not what happened. this is more like "there's nothing wrong with trying to help your son in his career and getting him a job."
Berkshire Hathaway's market cap is a trillion, and Warren Buffet's net worth has never been close to that, i.e. it's not his company so he's not leaving it to his son to run.
You might admire Warren Buffet's acumen and think he'd be good at picking people, but all (for varying values of all) parents believe in their children so it's not easy to tell if you'd be getting his best advice in this case if you were another major shareholder in Berkshire.