Warwick and Skarner are both super vulnerable to Quicksilver Sash. Warwick especially needs a massive rework because his entire kit revolves around his ult, which is countered by a cheap commonly purchased item.
Poppy is overpowered but her laning phase is horrible. She generally needs a decent rework as well. Fiora and Vi are fine, but Fiora is just a feast or famine kind of Champion meaning she either snowballs really hard or falls miles behind.
The problem with Zed is that he is resourceless, has an incredibly safe laning phase with both poke and easy cs'ing, in addition to strong wave clear and good gank escape. He was impossible to dive under his turret but could easily dive his opponent without taking more than a single shot. All of those things meant it was hard to make Zed fall behind in lane while it was piss easy for him to start snowballing due to his dive potential. None of the Champions you listed have this kind of deal going.
Zed's nerf was really clever in this regard, because it only reduced his snowballing potential without taking away his identity. It didn't reduce his damage potential, it just made him less safe and more risky to play like almost all other assassins are anyway.