Not unique? He has some pretty rare moves like coil and extreme speed. Access to extremespeed in this priority heavy metagame alone makes him worth a look. He can beat every priority user I can think of
Aqua jets from Azumarill can 5HKO (18.7-22.6) an unboosted Zygarde (which is rare since Zygarde can coil very easily considering his bulk), while Earthquake 2HKO's (52.7-62.4) Azumarill. Azu's Play Rough 2HKO's (choice band sets do OHKO though) Zygarde , but since Zygarde's faster, Zygarde still wins in most situations
Shallow? Yes. Bad? No. He has 2 great boosting moves that make up for his below average atk, and access to earthquake, extremespeed, stone edge, and crunch (yeah, that's pretty much it), but only 9 pokemon (Breloom, Virizion, Chesnaught, Torterra, Bronzong, Claydol, and Flygon, and only the last 3 can really hit him back if he gets a few coils in) resist earthquake and stone edge, meaning team support isn't that difficult. He doesn't have a lot, but he has what he needs.
Besides Kyurem, 600 is the highest BST in OU and is higher than every non [psuedo] legendary there is. How is that terrible?
weak to ice, dragon, and fairy, 3 common offensive types, but if you had read the thread, we had already discussed how he can beat salamence, dragonite*, garchomp, kyurem, and every fairy/ice type 1 on 1 almost every single time. That basically leaves Noivern, Hydreigon, lati@s, and floating ice beams that can really hurt him.
*we didn't fully discuss dragonite, while zygarde can beat a DD set, weaknite and mixed sets rip this snake apart
I think I've made it clear that he can hold his own and then some in OU, and there's really no reason he should end up any lower imo. Yes he has checks, but if he didn't he'd be banned to uber. I realize you were just pointing out some flaws in this pokemon, but would you please do some more research before making a statement like putting this guy in UU. He's a fantastic physical tank, and an even better late game cleaner. And most importantly, he's pretty easy to fit onto a team. I'd say that zygarde's only real negative attribute is that he's banned from vgc (land's wrath woulda been awesome in season 2).