I can appreciate why inexperienced lifters are encouraged not to try round back deadlifting until they are much stronger: their muscle control is very poor, and teaching them to remain rounded but tight would be challenging, to say the least. The risk is that, starting with the upper back rounded, the upper back will round even more, muscles having relaxed, putting all the stress on the skeletal components. Therefore, my initial impression is that the round back deadlift in some ways requires more upper back control and strength that its flat-backed counterpart (those of you out there who've done it this way, would you share this opinion or not?).