E-AA-W takes over a second of time, and leaves you quite vulnerable to being CCed during that time, as E self CCs and roots you during its wind-up and cast, and you are rooted during each auto attack. Q then takes another second, at which point, over 2 seconds in, they have only 3 stacks. You have less than one attack a second, so it takes almost 4.5 seconds to get 5 stacks on a non-moving target, who you are able to open up with an E on (disregarding all the time and damage you would take to even get in range to E) assumes no one CCed you, and that the target had no mobility, and did not flash, dash, blink, etc away, or block or dodge any of your hits, including Q's blade.
In an actual game scenario, it WILL take a MINIMUM of 5 seconds to get your stacks as Darius, against a single, caught out target, let alone in the middle of a team fight. Also, currently most Darius can be bursted 100-0 through Steraks in under 3 seconds, and unlike Irelia, he has no movement skills of any sort let alone ranged, resetting dashes with three ways to reset that also heal him for casting, nor any defensive steroids let alone one with massive %damage reduction and then burst damage (while also giving a free stack twice if you hit the last part) to help him survive getting in, and actually stacking up. Also, he can't pre-charge his stacks on minions, proc conqueror and his full passive with a single auto attack, then burst you down in a moment while you're CCed the entire time.