No really, why do people think Greg is OP? He's definitely fast, but it's not like he has all of these crazy strong and safe moves. Water shurikens take a while to charge (nor can you save the charge), side b can be heavily punished, up b is good but takes skill to control, and down b can be punished even when you land it! Not to mention how the counter window is really small compared to others.
Let's compare that to Sheik now: needles can be saved and are easily spammable, down b can even be led into by the needles, down b is a safe recovery move, down b is a kill move, down b is practically completely safe on hit and you don't go into helpless after it, up b hits on start and on recovery, up b has a wind box when reappearing that will blow opponents away that don't get hit by the additional hitbox, and side b has a lot of utility but seems the most balanced.
Now even if we were to unreasonable say that Sheik and Greninja have identical regular sets, you would see that Sheik clearly trumps what Greg has.