I hate characters whose mechanics basically force you to use them. Riley's current attack sequence post-ult makes her unhittable for a significant amount of time, making her unbalanced for Arena. I know I get the same benefit from her as my opponents, but I just do not think it is fun.
Alao, I feel her Howl/Wolves thing does WAY too much dmg to back lines, especially since she then pounces into them right after to stun and do even more damage, all while being basically un-hittable. Her pounce should be interruptible like most abilities, or she should function like Esabel or Alana during those phases and still take damage. I think this would bring her more inline with other characters, and stop the meta from evolving into teams that ALL have Riley.