Allright, so:
Kino: Yes, Rengar is not too adaptable. He has a certain style of play which consists of focusing on bringing a single target down. And yes, if you don't take a few kills in the beginning, you will be weak. But then again, Rengar has sick early game. Jungle with him almost guarantees you will have good early: kill from a gank or counter jungle is almost guaranteed. On top, newbie players will just fail, mildly experienced players will use his enchanced Q an cry as their enemy escapes with 100 hp, and pro players will poke the enemy down from the bush until he is around 60% hp, then combo him in a deadly all-in for the first kill. And then again, I do not agree about the tankyness. He is not a tank, but he can have pretty decent survivability without holding down on dps (Frozen Mallet + Black Cleaver + Trinity Force - sick damage with enough survivability - add boots, and that leaves two slots open - one is for your BT and the other one for either a tank or dps item, or the necklace).
Vunjo:
Late game - you should just poke them with E until the teamfight starts. Then you activate ultimate and take down their marksman in a quick combo. If you used it correctly, the marksman and his support will be too occupied by the teamfight to notice you until it is too late. Then, without their marksman, with the teamfight raging on, you can safely commit to taking them out one by one. No, this is not guaranteed to work. But it is not useless either.
Tankyness - already explained that to Kino. And the roar is actually more useful in top lane trades than you might think.
Assassination quality - well we disagree here more than anywhere else. Maybe Yi will do more damage, and Fizz, Talon and Zed will burst whole areas, but they have a crucial weakness - you see them all coming and can react. If played right, Rengar is guaranteed to burst down a single target in a materr of a second.
The part when you mentioned he is a bad bruiser - well yeah he is an assassin... Anyway, if we are in the stage of the game where the adc bought their tanky items, meaning their final builds, you can only expect the same from Rengar. This means he will be good enough to destroy them in a second.
The last part - In short - if you win the game, you will win the game and it depends on the player... I am so fascinated! When did you discover this?
Just kidding mate
But come on, that is so obvious and same for every champion.
Lord Demon X
He is quite dependable on using your abilities at right times. He might not be an easy champion, but he is a damn good one.
The ultimate is an awesome tool. Also, you do depend on using it at the right time. And it can be wasted. Just like 90% of other ultimates. For instance, you fire Ez's or Draven's or Malphite or any other not-single target ulti and miss. Failed hard.
Necklace. Yes. Can be an awesome weapon, but can also be a waste of money. Well, Rengar depends on the player's skills more than any other champion. In good hands, he is extremely lethal, but he can be failed quite easily. Just to make it simple > Rengar benefits from the player skill much more than any other champion, but that can go the other way.
Yeah, I agree on this. Twisted treeline is definitely better for him. This goes for Dominion too. In the Crystal Scar he can easily find isolated targets and leave them without a chance for escaping.