its that due to the current meta of picks/champs items with certain champs he got left along the way, he can be ok in the right pick/match up. but yes he can do tons of damage and back 2 seasons ago i got so sick of dealing with him top cuz he was hard on the champs i played that i picked up YORICK just because of jayce. last time i saw a jayce top i picked yorick into him and he hated life. jayce cant tank, he dont have any real engage, his knock back is ok for some disengage. but he isnt gonna flash in knockback insec lee style in the middle of a team fight. when you have a squishy support, a poke adc and a poke/supportive control mage picking jayce with that creates a gaping hole in the team comp.
but correct me if im wrong i never have played jayce but he seem he would be bad vs champs that can dive on to him such as
and they are regularly picked in top at this current state