Kha literally does NOT have to be in melee range. Leap has a 900 Range, and 700 pre evolve. 700 range counter jungling is a sure kill even if warded. And then he has a followup 1000 range on his Void Spike Slow. In other words, and his Q's range is 325 his effective range is for a kill can be up to 2000, the longest of any melee champion.
His damage passive ensures kills post Duskblade, resets, and his stealth near guarantees success.
You're obviously not a jungler or solo laner. Kha dominates junglers, Tops, and Mids, and they don't have any choice but to be Isolated (at least mid and top has a turret; Jungler is at the mercy of warding a bush near Red/Blue, Finding no Kha there, starting red/ Blue, and then when it's 40% HP getting 70% hp'd to 0 % Hp'd by Kha jumping from the other side. And then he owns your jungle for the rest of the game, even if you sense his wards.
http://champion.gg/champion/Khazix/Jungle? Look at his Kill vs other jungler stats.
He only has a 50% win rate because although he kills, many Khas never leave the jungle and use the advantage to push turrets. But he tops out in Kill averages, and players with over 200 games have an AVERAGE 60%+ win rate.
There isn't an Elo that exists where Kha isn't right at the top of the Kills per game graph.
And, the charts are skewed toward the fact that while players who have 201+ games AVERAGE 60+%, most Kha players have 7.25 games (He's just not a favored champ aesthetically) If we're talking an actual Kha player, they are King of the Rift.
You can talk all you want about not being Isolated, but it's one of the easiest conditional damage boosts in the game to meet.
You can dodge Zoe's bubble or trap 70% of the time. You Can't dodge Isolation.
This is where Zoe is on Plat +..... the players who are best apt at grasping and abusing a new champ:
http://champion.gg/champion/Zoe/Middle?league=platplus
She's still tiny in EVERY parameter.
4th from lowest gold.
#1 in Deaths.
ranked last in minions killed.
rank 30/46 of champs damage taken
Only Rank 12/46 in damage dealt.
And it's not a small sample size.... 31.55% of Platinum + games atm are choosing Zoe.
This is what Pros are doing with Zoe....... none seem that impressive:
http://www.probuilds.net/champions/details/Zoe
It is early and people are learning, but Plat+ are good indicators of where she'll end up, and it doesn't look too good.