The world shits on father's, mother's have to be extremely unfit before they loose.
The guy I worked with couldn't obtain as little as shared custody of his kids, she got full custody even though he was the sole income, had a stable job, clean records, and the kids wanted to be with him. While the mother was on anti-crazy pills, and frequently stopped taking them because she felt fine and didn't need them anymore, she also had quite the police profile due to altercations while off he meds. Child protection services took the kids from her many times, only to return them later. The kids moved in with him when the courts couldn't force them to be with her anymore.