Many champions, like Mordekaiser, Sion, Aatrox, and Shen, have a mediocre base HP. This is because, according to Riot, they have ways to increase their effective HP: Mordekaiser has his shield, Sion has a passive on W that lets him gain infinite HP, Aatrox has sustain, and Shen has his passive.
However, while Darius doesn't have a shield, he has sustain on his Q. He can increase his effective HP by a lot with that sustain. This, coupled with his insane base stats, prevent him from being duelable early.
Not only that, but Darius has insane base stats other than his HP. His base armor is the third highest in the game, his health regeneration is the second highest in the game, his base AD is the third highest in the game... In fact, only his attack speed is bad. However, this is not a big deal since Darius is a caster, and he can reset auto-attacks with his W. He can still apply Hemorrhage at a reasonable speed. Another of his bad base stats is his mana regeneration. However, with his mana costs being low and the fact that mana costs become less relevant as the game passes, this is once again not a big deal.
Why champions like Mordekaiser, Sion, Aatrox, and Shen, must have bad base stats to compensate for their kits, while Darius himself doesn't have his base stats gutted due to his kit?