I have played 15 on many occasions, and even at grass roots, club and university level it is a devilishly difficult position. Get your positioning wrong and the oppo kickers have a field day getting territory and field position. If you are ever so less than confident fielding kick chases and taking big hits (and hearing the stampeding masses coming at you is off putting) or you simply are half a second out either joining the defensive or attacking line it can be costly.
It is why Eddie Jones recent guff about being able to put a player anywhere shows a true lack of understanding from the man, and also equally how specialists in their position are worth their weight in gold!
Umaga, Atkinson, Watson, Kibirige have tried their collective best, but they are not full backs and it shows. I wonder whether the era of a utility back is coming to an end, because put any of them up against a player dedicated to that position and they will be found out time and again. In rugby it is the smallest of fractions that can make the biggest difference, I.e how Kibirige fielded the kick by Lyon near our try line last week, and experienced fullback would of likely positioned themselves better, been aware of where they were in the field of play and importantly the oppo, and would of made better decisions.
We need a specialist fullback as a priority.