Thank you for your feedback and insight although I completely disagree with you on most of your points, aha. With the ultimate I think you misunderstand how lethality and damage works.
Also, what games are you in where you already have a Youmuu's Ghostblade at level 6? If you have a Youmuu's at 6 minutes then you most probably have 10 kills which sort of throws your argument about damage balancing out the window. You're practically saying "Wtf this Veigar with Luden's Echo level 6 is one-shotting a Syndra at half health". Like... how does he have a Luden's this early without being massively fed?
Passive may seem small, but is actually really overpowered.
Whether or not a passive is overpowered or not usually revolves around the rest of the kit and what items they buy, not just one thing. Is Fizz's E overpowered? It is a dash, can be used to cross terrain, does damage, slows, and makes him untargetable. Of course it isn't overpowered because it is balanced around the rest of his kit.
Although, I think Khurdan's passive is strong, it most definitely is not overpowered. Before 5 minutes and whenever your opponent backs they will have almost no gold. So if you manage to get a kill, the gold stolen will be so small that it will not be noticeable. Your opponent would need to have over 1000 gold for it to be considered a substantial amount where they lose 50 gold and you gain 100.
Then compare this to Twisted Fate's passive: he gets 200 gold just for killing 50 minions. Draven's passive: he can get 350 bonus gold off just one kill with 150 stacks.
Hell, if you showed me Darius's passive before he was released I would have said it was overpowered. That thing gives 30 attack damage level 1 and 200 attack damage level 18. That along with the bleed affect which can apply Cleave stacks and doubles his ultimate damage is good throughout the game. Compare this to Khurdan's passive which doesn't give any damage or utility and is only useful between 10 - 40 minutes.
Q = Katarina's Q, E, W all in one ability.
They're completely different in the way they work though. I just think it's unfair to say this is "Kat Q, E, W" when their damage, interactions, costs, and cooldowns are completely different. Especially with the way Katarina's passive works; it makes these basic abilities stronger than they appear.
If anything the ability is similar to Lee Sin Q. Except instead of a dash it is a blink. To make up for this it does less damage overall and Lee gets to decide whether or not to reactivate the ability whilst Khurdan is forced to go in. Let's not forget Lee's Q grants vision too.
W = Fizz dash. (did i mention this guy has way too much mobility) (will most likely have invincibility frames, no bueno.)
He can only use it only enemies below a certain percent of health so it isn't the same. It also has a 1 second static cooldown so you can't just spam it like a Yasuo dash. It doesn't apply on-hit and spell effects like Fizz's does which is what gives Fizz's dash its power when coupled with Lich Bane. Also you can't use invincibility frames as an argument.
Both Khurdan's forms of mobility are extremely situational, probably the most situational out of any champion in the game so I don't see how he has "too much mobility".
R = Passive is insanely broken. Lethality is super meta right now, and combining this with Youmuus, Duskblade and Edge? Insane powerspike and guaranteed oneshot on any squishy. Too much damage. 400 + 150% bonus ad is literally a oneshot on any squishy with enough AD. I'm assuming this guy is played mid, where the average health at level 6 is 1000, give or take. With Youmuu's, this guy's AD will be 137. He will do 406 damage with his ult. Combined with his lethality passive
Yes the ultimate passive is strong but remember, it only gives 20 lethality when your W is rank 5 with 0 current charges and you have 3 ranks in your ultimate.
The lethality is only 2 per missing charge so a maximum of 6 lethality level 6 because you can only have rank 3 Sunder this early. Lethality scales with levels so it's not going to have that much of an impact this early on, especially when it requires you to have 0 charges to get the full amount. Like... you can legit get more lethality from red runes.
Compare this passive to the one of Zed's. He can get like 60 bonus AD for killing a full item Twitch. This comes on par with Khurdan's ultimate passive except Khurdan only gets the bonus by using W charges whilst Zed gets this AD permanently.
I don't think you understand how bonus AD works. Bonus AD IS NOT the same as AD. Also not sure how you are getting a Youmuu's level 6 but okay. The ultimate only does 200 + 150% bonus AD at rank 1, which is actually not that much considering it's a skillshot which can be dodged.
Like... you will only have a Serrated Dirk this early into the game so your bonus AD will be around 40. Your ultimate will do roughly 200 (+60) damage not this random 406 you have. Then after damage mitigation it will do a lot less, probably around 195 damage. So these numbers you've come up with are complete shit and totally off.
Hell, I'll even do the damage calculation:
If you're against a Syndra she will be around 50 armour level 6. In terms of lethiality, assuming you get the full amount of 6 from Sunder, 6 from masteries, and 10 from Serrated Dirk. You will have 22 lethality which is 16 armour pen at level 6.
260 x 100/(100 + 50 - 16) = 194 post mitigation damage
That's not a one-shot, buddy. Not even close.
Okay at this point I can see you don't know much about this game given your "feedback".
Oh, one more thing. Impossible to gank because of E stun and Q mobility. Would be better if Q targeted champions, even if that got rid of the skillshot.
That's wrong though. If you gank Khurdan before he is level 9 he won't have rank 5 Sunder yet. He therefore cannot dash through you as you are most probably above the damage threshold for him to use the ability. With that is mind, he won't be able to stun you with his E, which also happens to be a skillshot. With your logic you can never gank a Syndra when she has her E up.
I'm not sure how his Q helps him escape a gank when there are no enemies for him to use it on when retreating to his tower. Bear in mind the ability has a cast time and takes time to travel to its destination, during this time Khurdan will most likely be dead.
You do realise there are champions that are picked for their safe laning phases? Not every champ has to 100% die the moment they get ganked. Some people consider Kassadin ungankable so they usually avoid doing so. Does that make him OP? No. It is just something you have to play around.
While not apparently the best at trading, he will almost always win lane because of his passive.
Put Khurdan against any ranged champion or mage and he will lose, he has no defence so will just be poked out of lane. He won't be allowed to farm as he has no way of farming at a distance so the odds are he won't even get a kill. He can't win lane off of his passive because the power from his passive comes from him getting a kill, not from before getting a kill. So the whole "almost always win lane because of his passive" doesn't even make sense.
but his kit is overloaded
Overall I don't think his kit is overloaded; in comparison to champions like Ekko it almost seems underloaded.
