Despite what everyone says, Darius isn't an easy champion to be successful with.
Please read before you instantly mash the downvote button.
At low levels of play, darius will probably get fed on account of people's lack of judgement, positioning and care in lane. Darius's playstyle doesn't appear to be difficult, (no skill shots to land, etc) but it actually is a very challenging kit to be successful with in any sort of real competative play, for a few key reasons.
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He has to be built correctly.
People's first instinct is to build damage on Darius. They look at his high ratios and his damage output and they think "I should rush bloodthirster!" But (unless you're in low elo) darius will have 0 success building damage. His kit isn't built to quickly deliver damage like an assassin. He needs 6 stacks of his passive for his ult to charge, and he has no lockdown cc to keep people in place while he fights. He has no innate tankiness to survive, and no AD scaling shield to keep him healthy to deliver his damage. You HAVE to build tank on his kit, maybe with a splash of damage, but primarily tank. It seems counterintuitive, but it has to happen.
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He has low range and no mobility
Contrary to what people think, most of the hardest champions in the game to play are ones without mobility. Mobility is a fail safe that offers a lot of forgiveness for mistakes.
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He has the obligation to get fed.
His power is all shunted into his early game. Unless you get fed, you will be subpar all game. Yes, it's easy to get fed, but if you don't, you've lost an equally-weighted round.
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he's a top laner with no sustain, no ranged harass and no defensive steriods
This is probably the worst. He has no sustain in lane, which means if you don't kill early, your opponent will just outlast you. He has no defensive steriods which means that you're going to be less tanky than most people. And he has no ranged harass, which means no fallback pattern.
Darius's primary role is to tank, to soak and to peel. He has decent damage no matter how he's built, but in teamfights you primarily use your cc and your threat to get people away from your carries. You must use your pull not to get people in (unless they're out of position) but to get assassins and backliners out.
Playing darius in bronze is easy. Actually being good with darius on a competative level is hard. And I would argue that the reason he is underplayed is that he simply isn't worth it; a master of darius will probably be worse than an adept teemo or trynd.