So why Is Jayce not considered a good ADC?

He's tough, he great for gank assists, he provides good damage and utility. He can survive getting dived and he has good power spikes. He has some cones yes, he has low sustain until you get tear (Ezreal much) and he requires a lot of practice and discipline since he has two isolated kits that almost make him two champions in one. But you know what, he fits into a plethora comps without sacrificing much. Siege comp he's got it. Team fighting engage/disengage he has both in spades. Split pushing, his passive and cannon E eat turrets and creeps. Objective focus, he has enough raw damage and poke to take them fast or push the other team off.

So why after all that is he still not a good ADC pick? Simple, he lacks an escape tool only because Riot won't unbind his hammer fall into a skill shot. Give him that and the sky is the limit. If Riot would just make his hammer fall a point click instead of a target it would help so much no matter where he is played.