Edit: I'm procrastinating sleep and uni work so might as well double down and explain this in more depth:
Here's the deal: Unless you're playing against tanks or have an all physical/magical damage team comp (ex. Riven top Lee jungle Yasuo mid with Senna supp - soloQ is 'fun') it doesn't really matter what build you go on Kai'Sa because you're going to be dealing similar levels of DPS and burst; this means that if you're a chad Ezreal player who decides to build Trinity instead of Gauntlet you're unfortunately going to get nuked regardless of what the opposing Kai'Sa decides to build because her scalings are similar enough to the point where she'll do a similar amount of damage with both her Q/W regardless of what stat she invested in. In other words - Kai'Sa builds AP/AD and unfortunate squishy dies a painful death either way.
Why is this a problem? Simple - It makes Kai'Sa too good at everything when one of her drawbacks was originally supposed to be what stat she chooses to spec into (again, AP or AD). This goes a bit more in-depth but essentially there are three 'balance levers' Riot can theoretically use to adjust Kai'Sa:
The problem with Kai'Sa currently is twofold: For one she can easily get all three evolves off a 3 item spike (Muramana --> Rageblade --> Nashors - should still work AFAIK, definitely works with boots) which allows her to optimize without any real tradeoff; this was far more notorious with Stormrazor + Rageblade + Nashors (including 2 item Q/E evolve with just Stormrazor + Rageblade) which has since been nerfed. The other problem is that because of this and because of the scalings in her kit Kai'Sa doesn't really 'lose' anything if she goes into AP or AD because - regardless of what she goes - her damage on her Q will be compensated by her W and vice versa to the point where squishies will be recieving a mixed damage threat that can go invis and gets a shield along with Zhonya, summoner spells and all the extra peel her team has to offer. Seeing the issue yet?