It's been over 25 years since I played at school (in the pack, which will amuse some - strong legs!) but...
The ball must be put into a scrummage straight down the middle. And the front rows can't use their hands to get the ball. Yet, at least in first class games, the side putting in the ball invariably instantly wins possession.
I can understand how the team throwing the ball into a lineout (where the ball also has to be thrown down the middle of the two teams) usually win: they know where it's going.
But how are scrums so one-sided? Is the ball put in with enough spin to almost go into orbit? And why do they bother still having them given this?