You are right that he is not OP, but your statements are wrong and not helpful.
Darius is an anti-tanky DPS tanky DPS. Compare him to pretty much EVERY other bruiser in the game: He has no ranged harass or slow like Mundo,Olaf,etc. He has no gapcloser like so many of them have. He has no ranged slow/nuke like Nasus even. His grab is good but fairly short range.
As such, he has a big disadvantage compared to most bruisers vs any ranged opponents, including melee with ranged attacks.
To compensate for this, he is extremely strong vs enemy melee bruisers. His Axe is good short range harassment, his autoattack resetter hits hard and slows. His bleed adds magical damage to keep him from being countered by armor as hard, and his True dmg Ult counters the armor/MR bruisers usually stack.
Darius will usually win vs any kind of straight melee bruiser, especially the ones with gapclosers.
Darius will suffer against those melee champs with any kind of low-cost Poke, like Katarina, Shen, Cho, Malphite (if Q maxed), etc.
Darius will definitely have a hard time vs RANGED enemies like Kennen, Ryze, Vlad, and other potential toplaners.
Darius is unique in that he very strongly counters and is countered by the enemy pick.