Noxian might decimate and noxian guillotine are all balanced and are not whats wrong with darius.

Darius is only good in drawn out fights His massive slows on his crippling strike and apprehend allow him to turn extend any trade or fight to the point where he always wins

Rushing merc treads to reduce the duration of the slows and turn them into pitiful 0.50 second slows is pretty much suicide

your spending 1200 gold to buy a mr item against a champion that deals 100% ad damage

need to find a sweet spot on his cc where its not a insta death if you get caught or too weak to where he can't catch anyone

and as shown from a recent bug where his e had a stun effect we can't give him anything that restricts movement.