There are also internet forums about narcisist and abusive parents.

Each situation is different. And the people who is happy, rarely create forums to say so. What I mean is that I do know happy children with single parents. And adults that tell you that their lives improved after a divorce.

Since you can't know what would be better, you are forced to judge every situation on individual level. The idea that having a present father is always better is what leads to judges giving custodies to abusers and paedophiles.