She's a tad on the weaker side, the range between her scalings is too large. You can feel very underwhelming for the first 15 to 20 minutes of the game and suddenly become a beast. This is what makes her relatively inconsistent since you can hit these power spikes either very early or very late. This makes good Kai'Sa players a bit too strong and new Kai'Sa players a bit too weak.
I'm really worried she might get the Ryze treatment, where she becomes impossible to balance since pros will be too strong with her and newbies will be too weak. But ultimately LCS is the biggest money source for them and they need champions like this to make LCS more interesting..