Darius has what was usually the Garen issue: he's so well designed to ks that ultimately he can steal a ton of gold from champs that scale better with gold than he does. He falls off in late game, so all the gold he steal from early to mid game is ultimately wasted as the game goes on. It's just most people don't know or don't believe this, so they just surrender 20 minutes in when solotop Darius starts getting ALL the kills.
>falls off late game
>true damage ult scaling with ad
his late game is fine