While there are some who are arguing that Whis, Beerus, and MUI Goku are as strong as or stronger than Beast Gohan, it seems that his DP cost of 4 is what makes most agree he's so overpowered.

As Cell sees Gohan Beast for the first time, he says “W-what is this otherworldly strength?! Were Saiyans always capable of such unbelievable power?” Then Gohan Beast appropriately rubs salt ...