His skill kit is very simple to understand, but his playstyle is difficult to master. That is why a lot of people are terrible at playing him. People ask why he's hard when he seems so easy (aka right-click master), and I answer "precision timing" in teamfights. There's a small window of opportunity when Yi has to go in, kill high priority targets (feeder, ad carry, & ap carry), and get out. Miss that window, and he's either dead or a wasted asset that could be put to use pressuring another lane.