i already said it once. they should only give him the knockback effect on his hammer stance e, if he hits an enemy with his cannon stance q first.
otherwise its just free burst+disengage on an otherwise long ranged poke champion.
what does jayce do? he sits there and pokes with his cannon q throughout the entire lane phase. if you catch him out of position and engage on him, he switches stances and uses the movement speed from his passive and te knockback from his hammer e to run away so he can resume poking.
any damage you deal to him, he heals up by lifestealing on minions.
if youd remove the knockback effect without hitting the cannon stance q first, hed suddenly take a bit more risk to play.
jayce right now with his kit has almost the same problems old nidalee used to have.