he's extremely easy for your backline to kite and kill
Because he isn't suspposed to kill the enemy back line, the rework put him in a different spot than his old self, much better teamfighting but people still play with the old mindset and that's why he seems weak. He stay inside your team, you dont go diving the enemy team, you stay inside your team, between the frontline and the adc. Darius lack of mobility and intense melee damage means he wait for the enemy to come to him, instead of him trying yo come to the enemy and get kited. A diving assassin will get pulled off and killed because he's Darius, diving tank will have low enough mobility for Darius to get Noxian Might. Darius is one of the few tank-killing bruiser with Noxian Might, armor pen, Cleaver and true damage can easily beat up a tank, and spamming his W can keep most tank away from the ADC becase most tank also have low mobility. He is easy to kite, but he isn't suppose to be in a position to be kited. And he's still much tankier than old Darius simply by moving up a bit and hitting the enemy frontline with outer edge Q.