As a jungler she's fine (You should try Sated Diana it's good fun). However, as a midlaner she can be a bit overwhelming, especially against assassins, as she becomes this brick wall that you can't do anything about and will not only survive your burst but also kill you if you try to make any moves against her. Against mages, she essentially becomes old Akali who does absolutely nothing levels 1-5 but once she reaches level 6 she can instantly jump on your face and there's nothing you can do about it.
Personally I think the main problem with Diana isn't actually her kit itself. It's that being an AP fighter, she gets so much better itemization choices then her AD fighter brethren (same reason
was one of the best LCS toplaners for so long, he simply had better build options than most bruisers in the game). The juggernaut rework finally introduced some custom-tailored AD fighter items to help make them competitive, but their item options still pale in comparison to AP. Particularly
, it's an item that I think is frankly just unhealthy in the overall game and on top of boosting Diana's shield strength, it lets her survive shit that she otherwise has no right to be getting away with. They either need to create an AD version of hourglass or remove it from the game.