Anyone feel like diana is just not that Great right now ?

I always wished they reworked her into some kind of melee bruiser that relies on auto-attacks heavily. Less bursty, but more resilient, and a good candidate for . Currently the only champs building that item are Kayle and Teemo. Also Azir but he use it for the stats and not the passive since his soldiers don't apply it.

1m Mastery one trick speaking.

She's countered easily these days / AP amounts on items have been nerfed by significant amounts.

Banshees now has the old AP value of abyssal pre nerfs, but zhonyas (another core item) has gone from 120-100-80 AP. The Ap indirectly taken off of her core items affects how good she is. MR itemization is at an all time high, and with anti burst items like PD / Deaths dance, she becomes easy to kite back / kill after her first wave of burst.

Her most effective built currently is more fighter than assassin.

GLP > Nashors > Merc / Tabis / CDR / Sorc (whichever is needed) > Zhonyas, Abyssal, Banshees, or deadmans (depending on how heavy the damage is in the team comp, and what your team needs). > Void staff > Deathcap.

Tl;dr

More MR items, less AP damage, easy to kite / mitigate burst through itemization.

Definitely a low tier pick tbh

With the AP item changes she feels less battle and more assassin.

the 15% of this item (which is core in the battle mage fantasy) got indirectly nerfed cause there's ap missing from the current AP itemization.

That's the only analysis I can provide for now.

Currently all assassins have ways of quickly engaging and disengaging from fights. Diana differs from this by only have a strong engage and burst, making her much weaker in the current meta where getting into and back out of fights is key. Along with Riot slowly lowering the ap in the itemization, Diana has become one of the weakest assassins and an awkward bruiser because of CotC nerfs, which needed to happen. Diana relies on her high, rapid burst to kill her enemies before they kill her, and until she gets better ratios, or ap items go up, or riot reworks her E/kit to give her engaging and disengaging tools, she will remain weak everywhere.