Listen Jax was an absolute god before his rework nearly untouchable in the top lane and was an absolute 100% monster and was extremely volatile at the same time. If you didn't get dodges you would lose if you did get dodges you were nearly unkillable plus he also had a passive very similar to Vladamir where AD/AP would give him Health. He was the ultimate late game hyper carry that was running around with 3-4k hp before all of the tank items.
He's in a bit of a weaker state then some junglers but his kit is much less volatile compared to what it used to be. He could use a few stat buffs maybe but not much more than that as he's a champion that riot struggles to balance. He's a lot like Irelia a few small buffs and he's a god tier that no champion can match and it makes buffing him a bit more difficult personally I would love to see Jax close to his glory days but he's not really in a bad place and he's got a very solid kit.
His kit has got almost everything in it with the exception of true damage he's got a Gap Close, Damage Steroids, Damage Reduction, Complete Damage Mitigation, Attack Speed Steroid and all on very low cool downs. There really isn't much that can be done to him without tipping his balance one way or the other he's a tough champion to balance because his kit has so much to it and as I said before you buff certain things and he becomes a god that screams out "WHO WANTS A PIECE OF THE CHAMP!" as he bashes your head in with a lamp post.