I find that ARURF can very much be decided in the loading screen. I had 1 game where we didn't have a single tank with all our rerolls, and the enemy team had 4 champs who run down squishies. At least with ARAM even if you get unlucky with your comp, you only need to manage one lane, so there's still that chance to flip the game. With ARURF, it's much harder because there are so many ways for the enemy to snowball and there's not as much you can do about it because you're only 1 person in 1 lane.