his Q has a sizeable AD scaling, can still move while casting it, but probably worst of all, he heals a chunk of his missing hp with it... which only gets worse when he gets spirit visage... I'm not entirely sure why he needs the heal there, it's like riot is literally saying "just walk into the middle of the enemy team and heal up like 36% or so of your missing hp every few seconds if you manage to hit a couple of blokes."
his W is an auto reset with like a split second slow... slightly annoying considering his kit revolves around getting 5 bleed stacks.... however i suppose this is tolerable
his E... ugh not only is it a fast enemy displacement but it comes with a BUNCH of FREE ARMOR PENETRATION, WHICH ONLY GETS MORE ANNOYING CONSIDERING THAT HIS BLEED WILL ADD BLACK CLEAVER STACKS, SO EVEN HIM HITTING YOU ONCE WILL MAX THE BLACK CLEAVER STACKS.
like.. really why the unholy hell does bleed proc black cleaver stacks now and why does he need so much damn armor penetration... it allows him to kill tanks as well as squishies.
his ult... sigh... adds 20% more damage per stack so it doubles the damage and deals true damage at 5 stacks AND it resets upon a kill and lets him instantly give 5 stacks with one hit if those conditions are met....
basically....
if you ult and kill one poor bastard
the rest of their team is going to very likely get face melted...
on top of that he's the typical bruiser, builds one AD item like black cleaver and then tanky, meaning that he's going to soak up a good portion of damage if you focus him, especially if he heals with Q's
BUT!!!
leaving him alive in a teamfight is also a threat because of him getting the 5 stacks and triggering his ult reset.
hell, even illaoi is not that annoying, you can completely just walk out of illaoi ult and she has no hard CC like darius has. she has her test of spirit thing but that only pulls a soul and has a slow, not a full blown area pull and she has to stop to cast Q. She chases with w, but she's displacing herself towards you. Darius hooks you and pulls you towards where his team is, where his team can then CC you with death.