Rugby league is an excellent skill development stepping stone. You take out set piece, minimise kicking, and the number of phases you defend for and what do you have left to work on?
Your tracking, tackling, passing mechanics, footwork into contact, and running lines (particularly those two over/under screen pass lines).
It'll never develop a good prop*, lock, and 9 or 10. It'll also misses a lot of nuances. But by god are they excellent at the basics.
As an aside, what you lose in space vertically from league, you make up for horizontally in union. Never let a leaguie tell you otherwise.
* With the exception of Nic Henderson. What a man.