Pretty much what it says. His abilities are pretty quiet and use transparent shadows and transparent dark particle effects. In a big fight, you can't see anything at all. I played a couple games with him and noticed a disturbing pattern:

When I was trying to play him strategically and optimize everything, I was dying a lot because I couldn't tell when things were going off, when they weren't because I was hit by CC, when my ult dash hit, and basically anything that was happening.

When I just facerolled my keyboard and spammed everything all the time without using any strategy whatsoever, I got a lot of double kills.

This isn't how a champion is supposed to work. It's just too hard to watch the battle AND the skill bar at the same time, and unlike all the other champions, he doesn't have battlefield feedback.