Offiah was a hit in RL very quickly, not after years of hard work. Most union international outside backs would be a success in RL. Halfbacks are less convincing - scrum half in union in effect plays like a hooker in RL. When RL signed RU halfbacks a lot of them ended up playing in a different position - Davies was never that good at stand off, but brilliant in attack at fullback.
Forwards aren't that similar either, with RL forwards being far less specialised. Union front rows would generally be useless at RL, as would RL props at union. RL has no need for beanpole lineout jumpers, and compared to RU has a massive number of 7/8 style forwards.
In any event, RL won't be poaching RU players any time soon - probably never again. The main aim would be to minimise the numbers going the other way, and the bonkers selection of Burgess as a centre will hopefully dissuade both RL players and RU clubs. If they do target RL, hopefully they'll continue to sign RL players rather than target those who could more easily transition, as they'll have more failures than successes.
I have no idea how Farrell ended up being a backs coach. His international experience at stand off was similar to his son - boring and frankly looking nothing like a playmaker.
