My name might not be Diana, but I am a Diana one trick, and I play her 100% as an assassin. Better off just changing her subclass.
In my opinion it's not worth building her recommended ROA+Nashor's first because it's just more effective to go more bursty and one shot people. Mana+attack speed are practically wasted stats when you could just go more AP and penetration, since the AP also makes her W provide a bigger shield... So the more AP you have, the "tankier" you actually are.
Also, AP items aren't REALLY designed for bruisers, and if they are they kinda suck at it. It's either sustain+poke mages, or assassins.
Abyssal mask is a good item I will admit, I do like it. The problem is that it's only really worth taking into heavy AP compositions, or when you have a lot of AP on your own team.
Most of the enemy mid laners and junglers I'm facing lately have been AD, and most enemy teams I face have 3+ sources of heavy AD damage. There is no armor equivalent of Abyssal Mask, pretty much the ONLY option for counter building AD is to build a Zonyahs, which delays other important power spikes.
IN CONCLUSION,
I think we AP users need more options in terms of armor. AP users can stack merc treads+abyssal mask+banshees (some even build spirit visage too, looking at you Vlad and Swain).
But where do we go when we're against Zed, Talon, Yasuo, Jayce, hell even DARIUS mid?
Or what do we do against teams with one AP champion where the rest are all AD, so it wouldn't be wise to build magic resist just to help win lane?
All we can do is rush pure damage and hope to burst them down with nothing but a Zonyahs as a lifeline.
Therein lies the problem with Diana, in my opinion. It's also a problem that I believe exists with Ekko, and Fizz, two "similar" champions (but to a lesser degree because they both have more reliable ways to escape). It's the shop that forces us to play a certain way, not Diana's kit.
A Rod of Ages is sup-par in terms of tanky stats and damage. I want more armor. I don't want to sit here wondering if I should upgrade my Sheen into a Icebourn Gauntlet, or if I should do something silly like build a sunfire cape late (that actually might synergize pretty well with her E, but I mean... I'd really rather have better choices xD).
So yeah, if they want me to switch builds and playstyle from bursty to more bruisery, they first need to give me some tanky items that are actually worth building.
And I repeat, the magic resist items are fine, I DO use them when against heavy AP compositions. The problem is, most games I am against heavy AD compositions.
One more thing to add to this long rant here, is that I have attempted to remedy the situation using runes. You know where that got me? At first I was taking Iron Skin and Second Wind, and Iron skin got removed from the game and Second Wind got nerfed. Next I was using Chrysalis and Bone Plating, both of which got nerfed.
ELECTROCUTE, however, hasn't been changed. Oh, and guess what, they also buffed ignite.
Well okay Riot, I WANTED to build tankier and play safer, but you've doomed me to play glass cannon because it just works better.