He's pretty awful.
His slow is a skillshot that gets smaller the further out it goes and misses a large chunk of space inbetween. It also travels so slowly that it's next to impossible to hit if the person is paying any attention at all unless they're point blank on you.
His mobility and hard CC are tied to the same ability and is able to be interrupted in mid jump completely removing any impact of the ability.
His lifegain is on every third auto attack, and isn't very much at all anyway, then the damage amp costs his health on each third attack and comes at the expense of not getting a heal.
So for this character you want to build attack speed, attack damage, tankiness, and lifesteal. Meaning you don't have enough of anything to actually be impactful and you still get blown up if you get hit with CC. You can get away with not building attack speed, only if you never fight when your ultimate is on cooldown. His second life is usually wasted as if he died once he dies again, and it rarely buys you enough time to jump out of danger.
There are better champions for everything that he does, and he doesn't do much of anything well unless substantially ahead, which doesn't happen unless you're laning against an idiot.