Problem is that Rock/Fairy typing is far, FAR worse than Electric/Water with Levitate Problem with Rock/Fairy typing (at least for physical walling) is that Rock typing adds some bad weaknesses and in this case being weak to Earth and Steel attacks is something that Diance doesn't enjoy. Fairy typing on paper makes it good check to physical dragons, problem is that x2 weakness to Earth make her really bad at this job. Carbink is considered really shaky Dragon check and Diance with same defensive stats is technically the same. For example it can't wall stuff like Salamence, Haxorus, Dragonite, etc. as long as they carry Earthquake (which... well, is great coverage move anyway on them).
+ 1 252 Atk Adamant Life Orb Earthquake Haxorus
90.79%-107.24%
+ 1 252 Atk Jolly Life Orb Earthquake Salamence
78.29%-92.11% (33% chance for OHKO with SR up)
So one great niche, which is the ability to check/counter physical dragons is gone. Or if any of those dragons decide to carry Steel move for Fairies, it looks even worse when Diance is x4 weak to steel moves ;). While Fairy typing is in overall good defensive typing, Rock on the other hand is horrible. And lots of other physical stuff like Azumarill, Mamoswine, Lucario, Scizor, etc. beat you as well on physical side. Although she can stop Talonflame (but Talonflame may start running Steel Wing if Diance starts to be popular, so even this niche may be gone) so here's that I guess. And I wouldn't call her that great even at stopping Fighting types, because Rock typing (AGAIN problem) makes her neutral against them. Like for example...
+ 2 252 Atk Adamant Life Orb Close Combat Lucario
88.16%-103.95%
So I really don't understand how people say that she should be able to counter physical threats well, while she unfortunately can't do it in most cases (although some weaker Fighting types and Talonflame are countered, so here's that). If Carbink is considered bad at countering/checking those, I don't see how Diance suddenly would be better at this, unless you hit with Diamond Storm on switch. Problem is that in most cases you'll be the one switching on them and with this slow speed you may not even get a chance to use it. Personally I wouldn't even consider physically defensive set unless you desperately need Talonflame and Conkeldurr check/counter in one. Although in case of Conkeldurr, if he has Guts and is statused Drain Punch deals ok 37,83% - 44,74% damage, which may even end up with 2HKO if you catch her on switch and invest a bit of EVs in Speed to outspeed her. So yeah, I'm not a fan of physically defensive set at all. Rock typing makes is IMO meh at this job. If Carbink is considered gimmicky with Physically Defensive spread, I don't see why it would be different here. Especially when her movepool so far looks really lacking. Maybe that wouldn't be even a problem, but look how she would miss those OHKOes if she had at least 20-30 higher base HP... but unfortunately she doesn't.