User blog comment:Nelraye/Gehenna, the Eldritch Horror/@comment-24090662-20140227064132/@comment-24501229-20140309165508

Channel time on ultimate is intended to prevent Gehenna from using his ultimate to escape from a bad spot, leaving his ally to die in his place.

Also, 10 seconds ult - 3 seconds channel = 7 seconds swap time, which is exactly the amount of time needed to finish your recall with the Phasewalker mastery. Again to prevent using this ability to save allies too easily.

I'm not sure which version was up when you commented, but it's 8 stacks right now. (I don't think I've ever set it at 10 stacks). Using Q as an auto-attack reset you can Auto -> Q -> Auto to build 5 stacks extremely quickly. Landing your W to the face would grant a terrify straight away, giving you the terrify proc already.