I'd argue with Evelynn having a nuke.
True her E does have 1.0 AP ratio with decent base damage, but it still deals AD damage.
It still can deal decent amount of damage, but nowhere near what I'd call a "nuke".
While her Q deals magic damage. So building just for damage often ends with sour taste in your mouth.
Second thing is, because she (imo) lacks a nuke, her damage comes from burst, which is combination of E, AA, Q.
Thet's often enough to deal good amount of damage, but what comes afterwards makes me sad. Which is.. Q-chasing.
All other assassins have high bursts which often kill their target. While Evelynn has to chase for half of maps lenght to finally get her kill.
And last one: Noone can argue with that, but Riot doesn't want Evelynn to deal better than decent amount of damage (where assassins should exceel at). First thing is her tank ability. It's low-to-no damage, CC that gives her a shield. And worse thing: she needs it. Her high base damage abilities also inform us, that Riot doesn't want her built as serious damage-dealer (cause why would we want to build damage which mostly comes from our base stat, instead building 4th randuin.. Yes i know i'm exaggerating)
:)