After about 16 years of observation two of their kids have graduated (one with honors) and the older they get, the more decent they have become, very similar to their parents.
What I learned: Kids will turn out like their parents. And poorly skilled parents can still raise decent and well behaved kids.
Who you are and how you treat people is more important than technical skills to get your kids to behave.
One issue they had was their kids stayed up too late, didn't get enough sleep and screamed a lot near the end of the day. They were late to school (in elementary level) because the parents were tired too.
I think this is objectively bad parenting... but I could be wrong.
Edit: since I got one down vote let me add > they scream at their kids regularly (daily, sometimes the normal tone), let them play video games endlessly without supervision, and many other odd things. Kept a dog in a crate permanently in the house where it crapped and screeched at everyone. The list is long... Look at my other comments, I don't want to keep piling things on, but do some pretty outlandish things as parents.
Or, rather, the actual most important “technical skill to get your kids to behave”, in the long term view is modelling the behavior you want to see.
I've turned my family around from some pretty bad behavior simply by changing how I act. If they don't change automatically to follow, I may have to point out the change I made, and that I expect them to as well.
I have seen hard working, successful parents produce lazy, unmotivated adults who are still living at home in their early to mid twenties with no plans to attend school or find any way to support themselves.
It's still definitely nature and nurture when it comes to producing functional adults as a parent.
Kids do not always turn out like their parents and we have thousands of years of written history to prove it.
But I certainly would claim "always", but I would say that if parents are absent, it's less likely they will copy their parents.
Perhaps that is the key difference? My friends spend time with their kids, even if it isn't "quality time". Maybe neglectful parents can't don't pass on good traits as easily?
Keep in mind, I like these people, odd as they are, we just don't participate in much of what they do/have done that I disagree with. Our friendship overlaps a nice quiet area in life where we barbequed together, and mostly relax and let kids play or school activities. They do sometimes overcompensate it odd ways, sending their kids to camps and overly involved in sports and outside school stuff.
But they are kind, helpful, hard workers. And despite the fact that they don't do many things well, they do spend time with their kids.
Maybe there's something valuable about time spent, even if it's not "well" based on parenting experts advice?
Again, these are friends of ours who we care about, but they did some things that seemed selfish. But again, their adult children and decent and so far well adjusted.
I have kids too, eventually you are done doing the baby-chores like diaper duty. It is exhausting at times, but it's not forever.