Diancie seems like a legit defensive threat in ORAS, as it's one of the best checks to Mega Rayquaza, Rayquaza, and Mega Salamence, all huge offensive threats. Diancie also got Heal Bell in ORAS, making a RestTalk set with Heal Bell and Rock Slide / Diamond Storm / Moonblast a viable option, and the only cleric that is not complete set up bait for Mega Gengar, as Rock Slide 3HKOes and Diamond Storm 2HKOes it. Furthermore, Diancie is a flawless counter to Pokemon such as Ho-Oh, Mega Blaziken, EQ-less Mega Kangaskhan, and Yveltal, as well as a good check to Mega Mewtwo X and Darkrai, and has very useful supporting options in Stealth Rock and Heal Bell, while also being able to beat most Defog Arceus with Toxic, while not caring about status itself thanks to Heal Bell.
However, lack of reliable recovery is a huge problem, and to make things worse, Diancie has bad synergy with clerics, because they are all Fairy-types, outside of Shed Shell Blissey, which is an option i guess. Still though, RestTalk + Heal Bell is pretty reliable imo, so i think that Diancie has the tools it needs to be a successful defensive threat in Ubers.