My theory:
There's much more space in union and hence players run more angles and switches than in league and at a faster pace. This means that shoulder charges are far more likely to be mistimed.
In a match for my union team this year, our outside centre ran a line angling back in, and the opposing centre went for the flying shoulder charge, mistimed it due to the change of angle, and ended up headbutting him in the jaw.
Broken jaw, blood everywhere, our centre missed half the season through injury.
Running as a back in union is a lot different from running as a back in league.