People think he cheats because he has like 10x the number of sus clips as someone like flusha and because he had really weird stats.
He had a 1.2 rating for like 3 months was accused of cheating and then the rating fell dramatically. There are also screenshots of the same email he uses for his steam account being used for accounts on cheating forums.
Personally, while the evidence may seem compelling, I'm not in the habit of saying he is a cheater until the game can confirm it. Yes VAC is useless but engaging in mindless speculation when the only person who can confirm whether he cheats or not is valve is equally as useless as VAC. I get why people accuse him but the whole "is he cheating or not" is basically a zero-sum game unless valve magically proves he is cheating randomly.
One thing is for sure, he is not cheating in Val.