Fiddle's ult question

that would actually be different from how other mobility abilities work in the game though, Aatrox Q for example can be interrupted by displacement, but something like a dark binding will not cancel the dash, instead rooting him in the end location, one example from a game I've played involved hitting an Ezreal with Enchanted Crystal Arrow while he was in the brief channel for arcane shift, the arrow did not stop him from blinking because the blink itself is uninterruptible but he arrived at his destination while still stunned.

Fiddle ult is actually comprised of 3 different parts, a channel, which is interrupted by hard CC and is necessary in order to activate the other 2 portions, a blink, which is uninterruptible after the channel portion and is treated like any other mobility spells for the purposes of CC, for the purpose of soft CC, which only impairs movement the blink is considered to be started at the beginning of the channel, and the damaging self targeting aura effect.

You can prevent the cast of mobility spells with soft or hard CC, but you can only stop an in progress mobility spell with hard CC, with the exception of blinks which are uninterruptible while in progress because a champion will essentially be untargetable between the start and end points.