A rugby flyhalf has his hands out in front and catches the ball on the run. With the flyhalf moving forward the whole back line moves forward with him. Very few league 5/8's are in motion when they catch the ball. Rugby halfs throw a hard flat spin pass out in front of the receiver who must be in motion to catch it. Most league halfs pass with their top elbow extended and the ball flicked from the wrists. The elbow position makes it impossible to follow through on the pass to give it direction, speed and distance. The wrist action causes the ball to float end-over-end in the manner of a dead duck. The end-over-end ball is more difficult to catch than the spin ball which is partly why league players are flat footed on the catch. Watch any NRL game and you'll se it repeatedly. Look out for Jamie Soward, particularly as he passes to his right for the dead duck phenomenon. My point is that League 6 and 7's, whilst generally good defenders and athletes, have major structural flaws to overcome in order to be successful in union. In the opposite direction, rugby 9 and 10's who can tackle can be very successful at League. Wally Lewis and Ricky Stuart spring to mind. I can't think of anyone going the other way.