Agree with your last point, antonius. Union forwards are a special breed, all the positions are specialized. Hooker needs to be short, squat and powerful for scrummaging and rucking, plus they need to have very strong arms to throw lineout ball. Your props need to be the same build as well, while your secondrowers need to be tall and good at lineouts, as they will be getting most of the ball there. Your flankers need to be very mobile so that they can get to the breakdown quickly and ruck if need be. League forwards on the other hand, need to be fast, mobile and strong defensively as they will be tackling all day. There haven't been many succes stories of union forwards moving to league and vice-versa. The last one of any note here in Australia was Garrick Morgan moving to league (that was a disaster for him).
Backs on the other hand have very similar skills in league and union, they need to be fast, evasive and be able to run into gaps when the ball is run through the backline. Maybe the union backs need to be better at kicking than league backs, as they do a lot more of that for positional play, but league backs are much better defensively IMHO.
Most of the cross-code moves have been from union to league, but in the last year or so some big name league players have moved to union. Andrew Walker is an ex-league player who is now playing for the Wallabies. And from the end of this year, two of the biggest names in league move to union too - Wendell Sailor and Mat Rogers. Jonah Lomu can start getting nervous.
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