1.Having no Item is a HUGE drawback,this means Zam-C cannot hold leftovers so every damage it takes won't brushed off meaning it cannot take repeated hits, Lack of Boosting iteam is also a hinderance since while 130 attack isn't bad its certainly not gonna be dealing to much damage to bulky pokemon or even offensive pokemon for that matter(With some execptions of course)and Zamazenta has to rely on the mediocre Howl to increase its poor damage output.
I would think that a lot of OU Pokemon wish they could hold a mega stone ("wasted item") to get a stat spread of 92 / 130 / 216 / 80 / 145 / 128 though, especially bulky Steel types. That's a BST of 791 with no inefficiently allocated stats (Kyurem-B's 120 SpA being often wasted next to its 170 Atk). I know BST's don't stand-alone win any arguments nor should they, but that's a ridiculously good spread for an OU Steel type.
It hits hard enough. Even when unboosted, it can OHKO Heatran, Kyurem-B, Urshifu, Pelliper, Excadrill, Tyranitar, Blissey, Blaziken, Tapu Lele, Zarude, Hatterene, Hawlucha, Hydreigon, Grimmsnarl, Crawdaunt, Magnezone, Mamoswine, Tapu Bulu, Cinderace, Kartana, Barraskewda, Ninetales-A, etc. Zam-C also outspeeds the majority of the tier, allowing it to only take one hit in exchange for 2HKOs when it needs them.
At +1, Zam-C OHKOs nearly everything that outspeeds him (scarfed Lando-T being the main exception unless Zam is running Ice Fang), and Zam-C with only 64 HP EVs (Adamant) can't be OHKO'd by any of them. He can also OHKO the more delicate hard-hitters while unboosted, and most of them can't even OHKO Zam-C even with super-effective 120 or 130 BP STABs (when Zam has minimal defensive investment at that):
252 Atk Libero Cinderace High Jump Kick vs. +1 64 HP / 0 Def Zamazenta-Crowned: 188-224 (55.1 - 65.6%) -- guaranteed 2HKO
252+ Atk Zamazenta-Crowned Close Combat vs. 0 HP / 0 Def Cinderace: 273-322 (90.6 - 106.9%) -- 43.8% chance to OHKO
252 Atk Life Orb Pheromosa Low Kick (120 BP) vs. +1 64 HP / 0 Def Zamazenta-Crowned: 257-304 (75.3 - 89.1%) -- guaranteed 2HKO
252+ Atk Zamazenta-Crowned Behemoth Bash vs. 0 HP / 0 Def Pheromosa: 384-453 (135.6 - 160%) -- guaranteed OHKO
So imagine how Zam-C takes hits he's neutral to, or god-forbid, resists (he only has 8 resistances and an immunity).
232 Atk Landorus-Therian U-turn vs. +1 64 HP / 0 Def Zamazenta-Crowned: 10-12 (2.9 - 3.5%) -- possibly the worst move ever
232 Atk Landorus-Therian Stone Edge vs. +1 64 HP / 0 Def Zamazenta-Crowned: 14-16 (4.1 - 4.6%) -- possibly the worst move ever
The dude reliably takes more from a quad-resisted Stealth Rock than he is from Lando-T's 232 Atk-invested (145 base attack) U-turn
without any defensive nature, any defensive EVs, and only 64 HP EVs. When he resists hits, he barely feels them. You can quadruple those numbers by giving him a normal typing and it's
still impressive how he's eating those up.
2.Close Combat its best STAB move lowers its defenses meaning it basically neuters its high defenses making it very easy to KO in return
Using CC once gets its physical defense back to +0 145, and a base SpD of 145 even at -1 isn't "neutered". But I mean, take one of the fiercest wallbreakers in OU with a super-effective 120 BP STAB move and a Choice Band and throw it up against Zam-C with minimal defensive investment:
252 Atk Choice Band Urshifu Close Combat vs. +1 64 HP / 0 Def Zamazenta-Crowned: 284-336 (83.2 - 98.5%) -- guaranteed 2HKO
252+ Atk Zamazenta-Crowned Close Combat vs. 0 HP / 4 Def Urshifu: 428-506 (125.5 - 148.3%) -- guaranteed OHKO
Zam-C also outspeeds (it only needs 124 EVs without a speed-boosting nature to out-speed a fully speed-invested/natured Urshifu) and OHKOs it (needing 0 EVs and no attack boosting nature to do so). So a Zam-C user could actually "oopsie-daisy" switch in a non-defensive Zam-C on a banded Urshifu's super effective Close Combat, guarantee-tank that hit, and then guarantee-OHKO it back the next turn without any attack investment, even when ignoring the CC droping Urshifu's defenses. I know "one example isn't everything", but that's a horribly non-ideal situation for Zam-C to switch into a Banded Urhisfu CC, and still comes out like a champ. Imagine it when it's used for favorable or neutral matches with OU-optimized EV spreads.
besides Zamazenta has very few oportunities to boost its attack due to its lack of recovery to switch into attacks
I think it's highly doubtful that a Steel type with 92 / 216 / 145 bulk and 130 Atk / 128 Spe with two powerful STABs that can OHKO plenty of Pokemon while unboosted will have any trouble finding revenge-killing opportunities for one boost. Its quadruple-resistance to Stealth Rock, immunity to poison, etc. is going to help it there too. Anyway, I'm not against testing it. But IMO people that argue "it's not great" are going to be proven wrong when it is tested. It
might not be
broken, but I'd bet good money that it
will be
great.