I'm happy and love my dad but he was not a great father in many, many ways. No parenting guru for sure.
Based on my own experience, I personally don't just look at whether kids are happy or not as adults in terms of whether to take their "advice" on parenting... I know lots of wonderful, well-rounded, lovely Christians from very difficult family situations. And I know of wonderful Christians who truly follow the Lord and sought his will in their family life who have children who have "strayed" as adults. There is no guarantee that you will have great kids if you follow X or Y parenting method, and I think that is one of the humbling things about being a parent and how God teaches us through our parenting journey.