Why Sylas' design isn't close to the Kai'Sa controversy.
I feel like the main problem with Kai'Sa is that she's been something people've thought about for a long time. Kassadin's daughter being thrown in the void has existed for a long time, and the disappointed stemmed from the fact that she looked absolutely fine for seemingly being in a place that breeds eldritch horror, and that a void-corrupted human was what was expected.
Sylas, however, hasn't been a thought for very long, so there hasn't been too much thought on what has happened to him. He's just some buff dude chained up. Demacian prisons aren't the void, he's not going to be some monstrous horror.
That's just my thoughts, anyway.