Union forwards, especially props and hookers, find it very difficult to transition to League. League players have a different kind of fitness from union players. Union props are selected and condition for short burst of upper body strength and power, push in scrums, rucks and mauls, lifting in line outs, Union has a 5 metre rule, so no run ups. Ball park, in League the ball is in play for around 65 of the 80 minutes whilst in Union it's around 35 minutes. A while back the All Blacks had a KPI of allowing the ball to be inn play for less than 25 minutes.
Union Flankers transition to League 2nd rowers far easier than props and backs transition even easier.
Always a Bulldog