Building full ap on the giant hitbox with very unreliable damage outside of two melee range abilities leads to a very feast or famine experience. You either get so far ahead that your lack of resistances don't matter or you get ripped apart by any team with even the tiniest bit of coordination. Besides, AP cho builds a decent amount of health anyways so it'll more likely be a large buff for tank Cho than a nerf for Mage Cho.
Personally I build RoA and then tank unless I'm really ahead or their team has no hard CC. Even then though I'm always at a loss of what to build.
Just not that great anymore. Sure if you have other ap's on the team it'll be great, but otherwise a significant part of Cho's damage is true damage.
Since you have the slow on your Q the Rylais slow is just kinda ok. I kind of feel like if you need the slow you won't be able to use it, and if you can use the slow you probably don't need it. Without resistances I don't feel cho can survive long enough to utilise the slow on his spikes.
Again without resistances I don't see cho getting into range of a competent team and tagging someone more than once before dying.
It has armor and AP, but it's active isn't utilised by Cho the best. unless they have a key ability Cho wants to stasis for he won't find much use in it.
Imo Cho's problem is that he a horrifying aberration of Mage, Tank, and Juggernaut. I feel like Cho'Gath wants to be an AP Juggernaut, but there's far too much CC in his kit for that. When Cho eventually get's a VGU I hope they hone in towards a mage-juggernaut feel, with big spells with big cast times and big impacts.