Why is Darius so hard?

Am I the only one who finds Darius to be unbelievably hard to play?

His laning is very straightforward and I come out ahead more often than not... but once he goes out of laning phase I CAN'T DO ANYTHING. (it doesn't matter how many kills I got in the lane)

"Build the stacks in the teamfights on whatever you can hit closest to you... then when you get Noxian Might... kill everyone one by one."

I have never been able to pull this off at all. Maybe once in ten teamfights.

Fights happen super fast (people die left and right). Even when I do get stacks half the time its useless because I can't really get on anyone that matters (almost all popular carries these days have blinks, dashes, or some kind of mobility).

Any advice on how to actually play him and have impact?

Note: I find Renekton super easy to utilise by comparison in both lane and teamfights. But Darius... I can't make him work in teamfights; he is supposed to be... "easy"????