no they wouldn't, you've already stated "the "league" side would only ever see the ball to kick it off to the opposition." without any real reasoning behind that statement, for one thing that suggests you're saying the union team would play 100% error free which is and define a 'proper' game of union?
A proper game is one played within the rules/laws, just because a team doesn't employ certain aspects, or attempts to deflect away does not mean it isn't a proper game! You don't think individuals or teams in any sport attempt to stray away from their weaknesses when playing teams where they know the opponent/s have a distinct advantage?

With all due respect have you even watched union in say the last 10 years? Ever observed how Argentina in the 2015 RUWC played a different style that was passing orientated (and destroyed Ireland doing so), ever watched Japan who aren't the biggest side by any stretch of the imagination and their style of play and beat a fancied South Africa? So your point about SBW and SB playing centre or whatever is neither here nor there, the only players of much size differential is no necks at prop and the second rows.

All that said the main problem league players would have playing union aside from rolling mauls and certain technical aspects, would IMO be the tackling style, the up top grab tackle as is so common in the last 20 years and more is more likely to get you penalised. Absolutely anything around the neck gets pinged, relearning how to bend your back properly would be required for a large proportion, there are some that can and still do it every week but they are few and far between.

I just think you imagine a constant rolling maul/scrum/lineout fest and that simply is not the case, and you can combat a rolling maul by simply not engaging to the ball carrier (who will have their back to you for the most part), without which a maul cannot be legally formed, this will totally stymie some teams and defending them becomes so much easier. This is one example from a few years back explaining it. https://www.youtube.com/watch?v=ZGFDYIyxXUg This means a league team will not be put in danger and has the ability to stop a rolling maul without needing equally big men.

That's the thing about union, it has its complexities and nuances that can be used to negate teams and play to your own strengths, in league it's fairly simple in the tackle/contact area that there's nowhere to go, this does make that aspect easier for a union team to compete in some sense but other aspects of league are still significantly different.
So of course the union team would win and league likewise in their respective codes, but you keep saying that the union team would have the ball from 1st minute to last and keep the league team scoreless is patently nonsense and means you don't really even understand union properly at all or its laws nor indeed how to adapt tactics to suit your team within a game. I think you underestimate a league team to adapt which is disrespectful at best.

no they wouldn't, you've already stated "the "league" side would only ever see the ball to kick it off to the opposition." without any real reasoning behind that statement, for one thing that suggests you're saying the union team would play 100% error free which is and define a 'proper' game of union?
A proper game is one played within the rules/laws, just because a team doesn't employ certain aspects, or attempts to deflect away does not mean it isn't a proper game! You don't think individuals or teams in any sport attempt to stray away from their weaknesses when playing teams where they know the opponent/s have a distinct advantage?

With all due respect have you even watched union in say the last 10 years? Ever observed how Argentina in the 2015 RUWC played a different style that was passing orientated (and destroyed Ireland doing so), ever watched Japan who aren't the biggest side by any stretch of the imagination and their style of play and beat a fancied South Africa? So your point about SBW and SB playing centre or whatever is neither here nor there, the only players of much size differential is no necks at prop and the second rows.

All that said the main problem league players would have playing union aside from rolling mauls and certain technical aspects, would IMO be the tackling style, the up top grab tackle as is so common in the last 20 years and more is more likely to get you penalised. Absolutely anything around the neck gets pinged, relearning how to bend your back properly would be required for a large proportion, there are some that can and still do it every week but they are few and far between.

I just think you imagine a constant rolling maul/scrum/lineout fest and that simply is not the case, and you can combat a rolling maul by simply not engaging to the ball carrier (who will have their back to you for the most part), without which a maul cannot be legally formed, this will totally stymie some teams and defending them becomes so much easier. This is one example from a few years back explaining it. https://www.youtube.com/watch?v=ZGFDYIyxXUg This means a league team will not be put in danger and has the ability to stop a rolling maul without needing equally big men.

That's the thing about union, it has its complexities and nuances that can be used to negate teams and play to your own strengths, in league it's fairly simple in the tackle/contact area that there's nowhere to go, this does make that aspect easier for a union team to compete in some sense but other aspects of league are still significantly different.
So of course the union team would win and league likewise in their respective codes, but you keep saying that the union team would have the ball from 1st minute to last and keep the league team scoreless is patently nonsense and means you don't really even understand union properly at all or its laws nor indeed how to adapt tactics to suit your team within a game. I think you underestimate a league team to adapt which is disrespectful at best.