Titanic Hydra on Darius...
is NOT as good as other conventional AD items on him.
The main argument for Titanic Hydra being good on Darius is the fact that with the extra auto reset, you can stack blood quicker on your opponents and yes, I do agree that is worth something. However, the part that goes to complete waste on him is the enhanced autoattacks, since the bulk of Darius' damage comes from his abilities and his autoattacks are already strong enough with Black Cleaver and other items.
Before the Hydra AD nerf, it would've been a viable item on him, since it would've added a plentiful amount of damage to his Q and other abilities. But now that it only gives 35 AD, it's not really worth getting. Black Cleaver (and even Youmuu's) is in almost every way better than Titanic Hydra on Darius. Whereas Titanic Hydra gives extra damage on his autoattacks and stacks blood faster, Black Cleaver allows you to have increased sustain in teamfights (because of cooldown reduction) and more damage (because of more attack damage gain + armor penetration).
Youmuu's is pretty much the same way minus the extra health but replaces it with a really nice attack speed steriod, which allows you to stack blood faster. The movement speed will allow you to chase enemies and and let you land your Q more easily on moving targets. It's also great for the flat armor penetration early game.
Let's now compare it to Death's Dance. With the huge amount of AD gained from Death's Dance compared to Titanic, it's already worth considering against Titanic Hydra. Combined with the cooldown reduction and very useful sustain passive, you're probably going to last longer in teamfights with the increased amount of life you are replenishing rather than the flat health you are starting with at the beginning of the fight if you go Titanic.
That said, I believe that Titanic Hydra may be good in some situations, like when you are ahead as the enhanced autoattack can allow you to blow up squishies fast. However, on even grounds or behind, it's virtually useless compared to other items since they can offer so much more in these situations. In conclusion, Titanic Hydra is an okay buy on Darius, but simply not as great as other items.
ffs, and then can build all tank and not worry about a damn thing about his damage output because his ult is too much damage in one button that resets, 
needs a few to get her ult back, but not darius, he is designed to pentakill with 
insert random tank item here.
passive makes that irrelevant, and he has the assassin buff that his q range if hit, is shorter than e range, meaning he can pull the target if he hits the q. Can't run once he grabs you and 90% slow, you can't stop it...
15% missing hp and removes all cc High CD and no combat stats, 
12-36%, low cooldown, combat stats, HIGH ones at that, waveclear, and can double damage if hit with the axe head ,
are so squishy that they will pop easily when they misstep once and are useless the rest of the time.
is a great counter to him, but kayle is a niche pick now a days, and kinda risky if your team needs a tank.
and tell me that melees can't fight him. A good 
can give you hell pre-6 as well.
and then tank. It makes you very sticky.
This item definitely could use a change. Right now, it's better on melee carries/assassins than it is on champions that it was originally intended for. It also has a really inconvenient build, as sitting on new Tiamat and a Ruby crystal for half the game is how a bruiser/tank falls behind.