Some thoughts on Tera, mainly circling Kingambit, but I think the mechanic is ultimately the problem: Offensive Tera, getting Adaptability and potentially dropping one of your types, is mostly balanced. The instances where Kingambit gets a 2HKO on Dondozo with Kowtow Cleave and Kilowattrel gets a 2HKO on Clodsire are the natural result of getting a game-unique one-mon power boost, and the defensive downsides of losing a second type usually balances that out. If Kingambit loses his Steel-type, well, I was going to list all the negatives of losing Steel-type and then I realized the post would be too long.
All of my "This is bullshit" moments with Tera have been the result of what I think of as a defensive Tera, such as Kingambit changing his type to Flying or Fire, flipping his defensive type-chart on its head but maintaining both of his STABs while gaining a potential third STAB. Dragonite and Volcarona also engage in this, favoring Normal and Grass respectively (as far as I've seen). I also see (read: use myself because I'm an idiot) Electric Valiant for resisting Flying and getting STAB on Thunderbolt to hit Corv that much harder. This is less of a direct issue for more explicitly defensive Teras like Garganacl, because they're not often relying on STAB without the ST part like Iron Moth or Bax, but I've found that even these worsen matchup fishing. I'll often build teams with a sort of "I can reliably break two mons of X type for my sweeper to clean" mindset, when time and time again their last mon will happen to have the necessary Tera to shut down my team.
The defensive type-changing feels unchallengable in a way that, comparatively, Megas didn't. Altaria may have dropped its Rock and Dragon weaknesses every time it Mega Evolved, but it always kept its Ice weakness (even if it halved it) and reliably gained Steel and Poison weaknesses. Maybe you'll have another Steel-type Attack if that Skeledirge Teras into Fairy, but if it's Water you're just screwed, and there's no way to tell.
Every measure of balancing Tera without fully removing it I've seen smells just a little like a complex ban, and while I appreciate that we're trying to preserve the signature mechanic of the generation, I think it would be better to either fully ban it or accept as a community that we're going to be dealing with an extremely fishy generation.
All of my "This is bullshit" moments with Tera have been the result of what I think of as a defensive Tera, such as Kingambit changing his type to Flying or Fire, flipping his defensive type-chart on its head but maintaining both of his STABs while gaining a potential third STAB. Dragonite and Volcarona also engage in this, favoring Normal and Grass respectively (as far as I've seen). I also see (read: use myself because I'm an idiot) Electric Valiant for resisting Flying and getting STAB on Thunderbolt to hit Corv that much harder. This is less of a direct issue for more explicitly defensive Teras like Garganacl, because they're not often relying on STAB without the ST part like Iron Moth or Bax, but I've found that even these worsen matchup fishing. I'll often build teams with a sort of "I can reliably break two mons of X type for my sweeper to clean" mindset, when time and time again their last mon will happen to have the necessary Tera to shut down my team.
The defensive type-changing feels unchallengable in a way that, comparatively, Megas didn't. Altaria may have dropped its Rock and Dragon weaknesses every time it Mega Evolved, but it always kept its Ice weakness (even if it halved it) and reliably gained Steel and Poison weaknesses. Maybe you'll have another Steel-type Attack if that Skeledirge Teras into Fairy, but if it's Water you're just screwed, and there's no way to tell.
Every measure of balancing Tera without fully removing it I've seen smells just a little like a complex ban, and while I appreciate that we're trying to preserve the signature mechanic of the generation, I think it would be better to either fully ban it or accept as a community that we're going to be dealing with an extremely fishy generation.