Union has far more variety for one. In that, it actually has a meaningful set piece.
That's not actually an answer... you didn't elaborate at all on the question.
Scrums are contested in Union and the variation that creates is that you can either win a penalty off it or steal the oppositions ball. In reality though, you can actually still push in League scrums if you time it right (there was a push over try in the back end of the NRL this year - so it does happen) so the only real tactical difference is that you can get kickable penalties from them in union.
Just to clarify, I don't actually want to see the scrum go, but I don't think anyone here has provided anything close to a reasoned case for it offering any extra "tactical depth" to Union.