In-game when Heartseeker Ashe fires her ultimate, all that is visible is the particle effects of the ult, not the arrow.
1. Choose Ashe and select the Heartseeker Ashe skin.
2. Reach level 6
3. Fire Ultimate, observe that the arrow is not visible and only the particle effects of the ult are visible.
This makes the skin unbalanced from the normal default skin since it is hard to see and is unfair.