Thought I'd give my thoughts about Melmetal now that I got reqs, wasn't planning on actually being active this gen as a long time lurker but couldn't resist the Melm suspect test partly because of the new gen hype and partly because I don't think it's broken (everything here will only be taking into account pokemon that are B rank or higher as I don't consider anything below noteworthy enough to be mentioning, except for scizor because it sucks and itself for obvious reasons)
I don't wanna talk about the av/cb or any other lesser seen sets in too much detail since I don't see how they could be considered broken at all.
Av is good against offense like taking lele's focus blast and switching into shadow balls but struggle to break through it's checks and counters without the extra power and utility it's other sets provide (T punch can't even 2 hit ko pex or corviknight), while cb can basically 2 hit ko everything in the tier but you can easily pivot around it and it's the most prediction reliant of all it's sets, and both of these sets can be worn down pretty heavily during the course of a game with all the rocky helmet chip they have to eat and random attacks they're tasked to switch into.
Then there's the protective pads + t wave set, which is the 2nd best set imo. The thing that makes this set better than the others I've mentioned so far is the fact that it can win against a lot of it's checks and counters like pex, bro and corviknight, with Melm being able to t wave them and then potentially flinch them down with double iron bash, while still remaining threatening to most offensive mons with double iron bash + t punch + superpower. The problems with this set is that it can still be chipped down like the other sets discussed so far (though not with rocky helmet/rough skin/iron barbs chip this time) and that it relies a lot on RNG with the paraflinching, like Togekiss and Jirachi, and it can potentially get punished by these defensive mons with a U-turn/Knock off/Teleport/Scald if they get through the paraflinching. This set isn't too reliable at breaking these defensive mons either, with it for example to have a chance at breaking pex, you first have to predict the switchin and hit the t wave, then paraflinch it 3 times in a row with double iron bash and then you finally finish it off with a t punch. if pex breaks out 1 of all these times you get punished with a potential burn from scald or a knock, making you basically unable to paraflinch it anymore due to the rocky helmet chip. It also doesn't help that pex can switch out any of these times which are quite predictable, putting you in a potential disadvantage and letting the pex heal up with regen. This set can't do much of anything to corviknight, rotom wash and zapdos either, but overall I'd still say this is the 2nd best set with it's nice utility and value against many matchups.
And pretty easily the best set would be the toxic + protect set, and the only set I could see it potentially being banworthy for. This set is able to retain most of the offensive presence the other sets have while combining it with survivability from leftovers + protect, this and being able to cripple most of it's switchins with toxic makes this set the most difficult to deal with over the course of a game. That extra bit of recovery from leftovers + protect really matters a lot due to how naturally insanely bulky it is, those extra slices of health could be the difference between you being revenge killed or you surviving and koing back the opponent with it's still incredibly strong double iron bash. You can do pretty much whatever you want with the last moveslot, but Eq is realistically the best since it threatens switchins such as heatran, volcanion, Magnezone and pex, potentially even 2 hit koing pex. The problem with this set is that it gets hard walled by common defensive pokemon, it's basically useless against corviknight, skarm, ferrothorn and the best it can do against a predicted rotom wash or zapdos switchin is toxic and then switch out only to get completely walled by them next time it tries to do anything. Common offensive mons can also potentially take advantage of it too, the only thing Melm can do against Urshifu is predict the switchin, toxic and then switch out (it can't even protect because of urshifus ability meaning it can go for any of it's threatening stabs or U-turn) and the best thing you can do against a predicted kartana switchin is double iron bash once (not even a 2 hit ko) and then switch out (you can't protect to heal with lefties since SD exists). This set is also fairly prediction reliant from my experiance playing with it. Lets say your opponent has a zapdos and a heatran on their team, then you have to play 1 in 3 mindgames until one of either zap or heatran dies, and if you guess wrong you'll get punished pretty hard (you also still get walled by zapdos after you toxic'd it). Overall, I think there's enough both defensive and offensive counterplay for it to not be considered broken.
But lets take all these sets combined since it can be difficult to tell what set melm is running just from team preview. Since it can be difficult to predict exactly what set melm is running, lets assume you always wanna switch into double iron bash since it's easily the most threatening move at Melm's disposal and it's something every set is running. Offensive mons that can take at least 3 double iron bashes includes Urshifu, Kartana, Magnezone, Volcanion, Bulky Volcarona, Offensive Rotom Wash, Heatran, Aegislash and Victini while offensive mons who can take 1 double iron bash and then threaten out melm or at least put a serious dent in it includes Offensive Buzzwole, Offensive Zap, Blaziken, Drill, Lando T, Chomp, Offensive Volcarona and Crawdaunt.
And then for defensive mons who can take at least 3 double iron bashes includes Corviknight, Skarm, Bro, Pex, Zap, Pelipper Gastrodon, Defensive Rotom Wash, Defensive Buzzwole, Ferro, Defensive Lando T and Defensive Heatran.
The point with pointing all of these mons out is that you should always be able to build a team with at the very least 2 of them in it who doesn't lose to the exact same coverage move, so even if Melm has the right moveset for one of these checks, it can't fit all the moves it wants for the other one too, and even if it does it still have to play 1 in 3 mindgames since the Melmetals coverage moves is often too weak to 1 shot the switchin, meaning you can't just stay in and tank the hit incoming hit after you guessed wrong if you want melm to stick around for a longer time. I gave an earlier example with Heatran and Zap on the same team, but something like Lando T and Volcanion or just slapping a Rotom wash on your team can put the Melmetal in unfavorable situations a lot of times, where it and it's team gets punished a lot if they predict wrong. It's also a lot easier to predict the Melm once you know it's set, which isn't too difficult to figure out (cb is self explanatory, the lefties set clicks protect all the time and you know it's the lefties set once it gets chipped a tiny bit and the other 2 sets have similar enough counterplay). Another thing is that it isn't as easy as people make it out to be to safely get in Melm either, while it's defenses are amazing, it can't really take repeated hits, and almost every good offensive mon can threaten it in one way or another, while defensive mons can cripple it with t wave, scald and knock off since it really hates losing it's item.
I don't think Melm is limiting the builder in any significant way, and it simply rewards players for predicting right and playing around it's counters. It's quite manageable from a game to game basis from my experience as it's neither the most offensively threatening as shown earlier or the best defensive mon due to how easily it gets shipped down with all the hits it has to take with it's poor speed, and it can potentially take a lot of damage every time it predicts wrong too. It's a good pokemon that always provide value to a good player who find the right set for the team, not a broken mindless pokemon who just clicks 1 move with no counterplay and certainly not the end all be all of OU some make it out to be.
Taking everything I've said into account, I will be voting do not ban, but I don't have any problems if anyone would think it's too much for the tier as I can see the arguments against it too. (Also fuck ladder and Heavy duty boots)