Ryze ult is a channel that allows him to tp to a location within his range, and can be cancelled by cc other than slows I believe. Well I just played a game where he puts his ult down and zhonyas and it tps him. Why does that happen? I don't believe tahm kench can zhonyas while ulting and it will still tp, why is it allowed for ryze??
The spell channels, he does not. He is free to move around once cast. Any allied champion simply disappears once the channel finishes.
Tahm's physically channels his ult and gets interrupted if damaged due to it being too strong if he could just eat an ally and get out of a collapse.
It's an intentional balance thing (probably). For example, Aurelion Sol keeps rotating his stars when hit by a friendly Bard ult, but they disappear like when he's CC'd if he gets hit by enemy Bard ult. Exact same ability, but different effect based on which team hits him.
Why specifically for Ryze? I dunno.
Kinda like how 
's ult works.
Tahms ult is a lot longer range than ryze's ult
He channels the spell shorter than the spell actually channels, giving him a brief window to do something else before warping.
Hourglass synergizing with ults is nothing new. Morgana and Kennan have that as their main combo. Since Ryze is still a friendly unit when in hourglass state (just not targetable) he gets warped regardless of being in hourglass or not if in the portal.
because it's a channel to create it, not to use it
