Alright since I have an hour of free spare time, I suppose I'll use it to offer some more insight from the side that considers Zamazenta problematic/broken.
.....But before I do that, I'd like to preface how I value a mon's traits and have them tied to their effectiveness: Speed>Bulk>Power.
Speed should be fairly obvious; the ability to get moves off before the opposing mon can is an advantage 99% of the time, no matter how you slice it. It's why/how mons like pult/deo-s attain all the viability that they do. It's why Flutter Mane/Iron Bundle were among the first things to GTFO ASAP. It's why I believe Chien-Pao should NEVER be considered for a retest again at the risk of spawning a giant war, I'd rather have CHI-YU than it, even if obviously neither must ever come back under any reasonable circumstances. And finally/most importantly...Zamazenta has it in spades, with its speed towering above the whole unboosted tier outside of the aforementioned pult/deo-s, which lets it get moves off before the majority of what it's facing, putting what little set unpredictability it has to deadly degrees (read: if your current (bulky, non unaware) mon relies on status to cripple zama, and the latter happens to pull out a sub, the former's gonna need several turns just to break it, during which time zama's recovered all the lost hp back with leftovers; sub also blocks the knock that'd help against those leftovers). It's also what allows zama to run train over teams relying on booster energy mons to check it, once it's roared them out (smth it can afford to do due to how fast/bulky it is, more on this in a bit). And finally, it is what allows it to instantly seal the game the moment any specially attacking mon capable of withstanding a hit is out of the picture.
Power may seem strange to include second, but I'll only briefly touch upon this since it's not super deep/I really need to get into the main point of this whole post. Power is ofc important to proactively end games when/if applicable; however, if the mon can't put it to use due to a lack of speed/bulk, it's basically meaningless (aka the Rampardos theorem). For a more concrete and meta relevant example, Hoopa-U has an even higher BST than Zama, however it's not anywhere near OU proper despite having a decent amount of viability. Why? Because in spite of its titanic dual offenses and spdef, low-to-middling hp, def and speed ensure that it basically feels like....a regular mon to fight against; most strong physical attackers can crack it wide open, u turn or not, and even its special bulk only goes so far w/o reliable recovery (and its offenses aren't fully felt because it can only afford to fully invest into 1 of them at a time, due to how the ev system works; no, you're not unironically maxing out both offenses and sacrificing both hp and speed, don't even bother). It's especially apparent in this gen where most mons need external conditions for sufficient off the bat power (tera, weather, abilities etc). Zama itself has a...solid but not-off the bat overwhelming power (without boosting with iron defense for body press, or choice band for its close combat sets).
Bulk is where the meat of it all lies into; if a mon can't move before its competition (an exceptionally apparent issue this gen with the most absurd speed creep of any gen ever), it will have to fall back on its bulk one way or another to compensate, before being able to get moves off. This is not to say that glass cannons can't be/aren't viable (far from it), but the more bulk you have/the more you're able to preserve it, the better. It's why even various mid bulk offensive mons run boots this gen; so that hazards don't compromise whatever hit taking ability they might have, even at the expense of power granted by other items. And....it's arguably the single most morbid part about zama itself, and an aspect often frighteningly overlooked. Many people will say just have your offensive team use booster valiant/scarf enam to check it/cripple most of its hp with moonblast. Which...isn't wrong, for sure. But a variety of factors make this a significantly more complicated matter than it would first seem. For an easy starter:
252 SpA Enamorus Moonblast vs. 252 HP / 0 SpD Zamazenta: 284-336 (73.1 - 86.5%) -- guaranteed 2HKO after Leftovers recovery
Not even close to an ohko. And valiant obviously does less (but at least it has encore which in tandem with moonblast can indeed prevail). And these aren't light hits either; they're among the strongest SE hits the entirety of OU has to offer against this beast. On its "weaker" defensive side. And it's still not even close to enough. (on the lowest roll, zama has enough hp to sub again if needed which is obscene). This is not even taking into account the possibility of tera, which lets it take essentially 0 damage from the above moonblast, and boost up further/crush these fairies right then and there. And before one says yay you forced the enemy's tera, now HURRY UP and WIN! ....no, it's not that simple; many of the offensive teams employing these mons, or any other such similar ones, primarily rely on them to deal with fighters like zamazenta; there's only so much counterplay an offensive team can afford to fit to any one threat -much less one THIS bulky/fast- this gen. Meaning that, it's often a fine investment for zama to tera vs such offenses, considering its ability to so swiftly/decisively seal the game the moment most immediate counterplay to it is out -and with how little room for error there is against it, especially with the right set (or at least cripple the opposing team enough for the rest of its team to win). This is exceptionally fatal considering that the main counterplay to such fast boosting mons, aka priority moves, do not even work against this colossoal stat stick that's further boosting its defense to the point where they do nothing. And hell, assuming it CAN tera, what special attackers are there to reliably stand up to its might, while being able to cover most of its tera options? The answer is fewer than you'd expect:
252+ SpA Choice Specs Protosynthesis Walking Wake Hydro Steam vs. 252 HP / 0 SpD Zamazenta in Sun: 363-427 (93.5 - 110%) -- 62.5% chance to OHKO (draco does 100% minimum)
So yeah, you need something on THIS level of damage output to be able to reliably dispatch it. This is not mentioning how wake just dies to a +3 body press btw. Almost everything else fails to do sufficient damage in time, especially with a timely tera allowing zama to get even more free turns to boost further or just fire off boosted attacks and win. Granted, your own tera (aka most notably tera ghost -btw how many things would actually run this if not for zama/dnite? be honest) can also be used to try and steal back momentum from it, but again it's risky and can get even more complicated if you've already given it too many free turns -especially if it happened to have the right set vs the right mon to turn it into total setup fodder. Setup sweepers of your own are also a risky choice on the off chance it's indeed carrying roar -except for okidogi, which is admittedly v.good vs it, but even it fears an untimely crit, and also zama boosts faster than dogi does (especially if you were hoping to knock off its lefties first but it reveals sub and now your turn got wasted). And even then, ghost type mons/tera ghost ones are the mercy of crunch defense drops (without covert cloak); a very real danger/possibility considering the above argument about how many turns the mon is able to generate for itself. It is able to generate these free turns because -as I've been building up to until this point- Zama is able to put its sheer bst to much better use than mons like Hoopa U/Kyurem can, largely due to the above mentioned speed and bulk letting it leverage the rest of its traits to a much more dangerous degree (fewer weaknesses compared to the above 2 mons also certainly helps on this regard as well).
....Ok since I'm apparently running out of time now -I was just about done anyways- I'd like to make some final few disclaimers: Is Zama too overbearing to deal with, considering all of what's been said above? Well arguably not, considering it's still in the tier now after several months/most of the general consensus seems to be no. Does Zama bring nothing of value to the tier? Yes it does, with its ability to stfile most Kingambit sets a mon that I still consider dumb btw and also just offering a fast, bulky cleaner with all of its above mentioned traits. Would the tier be better off with Zama gone? Well that's smth neither me nor anyone can say for certain, however the strain the mon puts in battle on practice -at least for me- has certainly left me to wonder just a little bit. And lastly, why did I not touch upon the other sets? Because they're not as capable of accentuating the mon's problematic traits as ironpress can- they're good sets for sure and have their place/role on teams, but....ultimately, these bulky boosting sets are seemingly always the factor that reveals a mon's true nature- see Ursaluna Bloodmoon, cm stored power Magearna (and various other mons on its wake still on this tier, which have made the move stored power so debated over even after all this time), and many other examples across various tiers and gens. And it's also why I firmly believe Zama-Crowned must NOT be given a real chance either (basically take everything I just said about zama above and up it to 11), and it's why various ubers being discussed above shouldn't be, either (most ESPECIALLY Lugia/Giratina).
TLDR: Zamazenta's combination of immense (unboosted) speed and bulk lets it exhibit the rest of its traits to tower over most of the tier with a(n arguably) relatively alarming amount of ease, and while it may not always be able to win right away, it can make use of resources like tera to overcome its few would be checks and, once those're gone, it can seal the game almost instantly with its speed and bulk quickly denying physically attacking revenge killing attempts almost entirely. While it can't run all of sub/roar/heavy slam at once, having the right one of these moves/choice of tera against a would be check allows the mon to prevail easily, and become significantly harder to take on from there, considering how little consistent counterplay to it there is outside of bulky unaware walls (and yes you can argue the mon is useless vs fat, however a mon doesn't need to be broken vs all playstyles to be considered problematic, as I'm sure most people already know).