Champions are OP!? Or is it their Items

You know, I see a lot of threads about Riven, Yas, Rengar, Kha, and I think... well yeah, they're pretty damn powerful, but then I get into a game.... There is a Riven, there is a Rengar, they are being destroyed. Absolutely man-handled, you'd think a video about killing kittens would be more kind! Then... something happens, they get a core-item. Rengar suddenly deleted someone. Riven just ripped the top-laner a new ***hole. Its to the point, that I think the biggest problem in this game is less these champions and more the items they're buying. Duskblade is seriously still one of the most HATED(but loved cause you'd be an idiot to not use it on your assassin) items in this game, next to Spear of Shojin and they're only built by certain champs. They have so many boosts. You can take basically ANY part of Duskblade out and I assure you it still be bought almost 100% every time. Why does the person buying this need a mechanic to not only be alerted to a ward nearby(one that can't even be seen, nor has it seen them yet in some cases) but also able to one-shot it. Why is there this large amount of Bonus Damage being given for not being seen. Like, you have to be a total tool to not get this effect to activate on Rengar, Kha, QUINN, etc with their stealth(or unholy amount of MS) who already have passives that give them extra oomph to their attacks, especially Kha if he catches ANYONE by themselves. This item already yields 55 AD, 10CDR, and +21 Lethality... I honestly think the problem with most of these champs is simply the existence of these overstuffed with utility items.