Gav, it has always seemed to me that union players go straight to ground, in order to form a ruck and maintain possession. Whether or not this is submitting is up to personal interpretation - to me, not struggling is submitting.
The fact that league teams employ wrestling coaches says to me that they are not submitting, and that there is, in fact, a struggle. That was one of the points I was trying to make. It has to remembered that, in league,field position is determined more by forward running than in union, where kicking is the more important strategy. For a league player to submit as soon as they hit the line makes no sense - their job is too make ground.
I still think that it is more important to land correctly in union as, if you do not, there is a good chance that you will lose possession. That is why you see players go straight to ground, rather than the struggle that you may see in league.