I don't want to get into a Garganacl debate so please don't take this as me arguing in favor of Garg getting banned or anything, but I think that this logic is a little disingenuous. The meta has adapted to Garg via things like Covert Cloak (which is still almost always an inferior item that you're being forced to run in order to check Garg), Substitute, and Encore, but Garg is still excellent and was just one of the best performing mons in World Cup. There is enough variety to Garg sets (and the nature of Tera obviously exacerbates this, although I'm not taking a stance on Tera here either) that it's difficult to conclusively say that much simply beats Garg; for example, even something like Covert Cloak Gholdengo, which is presumably being used as a team's best answer to Garg, gets dominated by Tera Water Garg with Earthquake. Sure, there are some mons like Covert Cloak Gastrodon with Clear Smog that should beat essentially any Garg set, but you can find plenty of examples of irrelevant mons/sets that beat top tier mons (like how Dachsbun can pretty much counter Koraidon in Ubers). The problem with Garg to me is that it just feels incredibly "cheesy," kind of like Volcarona and Espathra did in OU; Garg feels more unhealthy to me than overpowered, if that makes sense. Sometimes you just get 6-0ed by a certain Garg set, and while you could say that a team should be built to answer the various sets it runs, this meta already has so many difficult mons/sets to check that it's putting even more strain on teambuilding and making it harder to check everything that you need to. I could be wrong about this but I think that may be part of the reason why HO/offense in general has been so prominent - it's easier to mostly ignore defensively checking so many things when you can instead stack threats that your opponent is more likely going to struggle to answer.
Honestly I don't think Covert Cloak is that relevant to Garg being balanced because it's a relatively unviable item otherwise. Substitute, Encore, Knock Off, Amoonguss, Gholdengo, Great Tusk, Kingambit, Meowscarada, Samurott, certain Valiants, your own Garganacl, and more all have a lot of value beyond beating Garganacl and one or more fit on (And in fact are ideal on) a huge variety of teams
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