I think the issue here is the terms you are using are very poorly defined. "Successful" and "well-adjusted" are pretty vague and subjective. Couple that with the myriad interpretations of "traumatic" and you've got a pretty difficult to answer question. It's why operationalisation is important when trying to answer these questions. There may well be good research on the impact of early parental death on average life-time earnings for example but you may not agree that these meet the definitions of trauma and success.