Ashe's ult stun outscales her new "Frost Shots" crit passive and doesn't synergize with it.
No matter how long the stun is, Frost Shots will only let you crit the main target of Enchanted Crystal Arrow within a 2 second window. You can land a great distance ult, but Ashe's ability to make use of the opportunity is ruined when she loses her crit, and getting it back again (by using Volley or attacking the target with a weaker shot) wastes precious time and puts her in a bad position. Meanwhile, ulting someone at a very close distance that would only stun them for 1.5 seconds, lets Ashe do all her burst damage with no problem.
Currently Ashe is encouraged to ult people at close range instead of landing those distance shots and maximizing the stun time. Remember her ult can scale up to a whopping 5 3.5 seconds, why isn't Ashe rewarded for running all the way there and engaging?
Can't enemies currently stunned by Ashe's R always count as being marked with Frost Shots?