BIG ANTI-TERA POST
I think the Tera's problems can be divided into two parts:
1: The pressure to account for all the different mons with their different Tera types in the teambuilder (see the Kingambit post), and...
2: The in-game mind games
1: The pressure to account for all the different mons with their different Tera types in the teambuilder
This is the one that I see less people complaining about, primarily because its symptoms (losing) can be attributed to playing badly or building bad teams. However, I think this one is the more broken aspect of Tera simply because there's very little way to get around it. When building a team in the Tera generation, you WILL be extremely weak to some Tera'd threat, and it's probably not even a gimmick set. The best you can do is account for the most common Tera users like Garg or Gambit and hope you don't run into something random like Tera Ice Sandy Shocks. Ctann already demonstrated how hard it is to build a team not weak to some form of Kingambit, but there are many common pokemon that abuse Tera and need different checks for different Tera types: Iron Valiant, Volcarona, Garganacl, Baxcalibur, Dozo, and Dragonite are just some of them. To prove my point, I'm going to analyze several OU teams that have been considered pretty good on the Teams of the Week threads.
Slither Wing Balance by mimilimi (wk 24)
Tera Fighting Gholdengo, Tera Dragon/Ground Bax, Tera Fire Dragonite, Tera Ground Iron Moth, Tera Ground Volcarona, Tera Rock Breloom
Choice Band Tera Flying Dragonite Balance by Pinkacross and Srn
Tera Fairy Garganacl, Tera Grass Moth, Tera Dragon Bax, Tera Flying Iron Hands, Tera Flying Bulkarona, Tera Fairy Ting-Lu
AV Slowking Balance by Mimikyu Stardust
Tera Fairy Skeledirge, Tera Water SD Valiant, Tera Water Azumarill, Tera Ice Shocks, Tera Bug Ceruledge
I limited myself to only OU (except UUBL Hands and UU Shocks, both of which are legitimately used) pokemon, and only types on the pokemon that are in the Tera Type Index. Even with those restrictions, you run into a startling number of Pokemon that can win 1v1 against the whole team aided by Tera (relying on bad prediction to beat them is unreliable.) I'd like to emphasize here that these don't need much, if any team support to break these teams if played well. If you notice a theme, it's that Tera gives a pokemon the ability to either set up on something that threatened it or break past a counter or check, which leads right into point 2
2: The in-battle mind games
This is far less of a substantial point but I feel that playing around Tera in-battle is extremely difficult. By virtue of all the different sets it gives everything, you have to pretty much be constantly guessing. For example, say I brought out a Volcarona against a team with Dondozo, Dragapult, and Toxapex. You have to predict whether it's the Tera Grass set, which beats Dozo, Tera Fairy for Pult, or Tera Ground for Pex. If you predict wrong, you lost a pokemon, which means in every battle you must balance the likelihood of the set vs the importance of that pokemon against the opponent's team, but often there is no good answer. If I had, say, Roaring Moon in the back, which beats Pult and Pex but not Dozo, then you would gamble on it being Fairy or Ground for Dragapult or Pex. However, this leaves you open to my Solar Blade Ceruledge in the back, common on the sun teams RM excels on. No part of this is competitive or balanced. However, we've had lure sets that could turn the tables on checks since gen 2's DBond Gengar, what makes this mechanic any different? Two reasons:
A: It has very little opportunity cost. If you have a matchup in which the thing you were trying to lure wasn't there in previous generations, you were down 1-2 moveslots or an item, which are significant. If you have a matchup in which the thing you're trying to lure isn't present in this generation, you can just not Tera and have your Pokemon work mostly as effectively as normally.
B: Everything can beat everything. In previous generations, there were guaranteed safe switches into things like Volcarona. Most fire types, especially Heatran, bulky waters, and some Dragons walled it. Sure, it could run coverage in HP ground or Giga Drain, but those were still pretty weak and took up a moveslot, meaning there was opportunity cost. And even if you didn't have a completely solid defensive Volc check, you could still be sure that your priority Aqua Jet from CB Azu would at least force it out. All those things go completely out the window with Tera. Your coverage is suddenly a lot more scary with STAB on it, meaning it will be used outside of that matchup, and you're never guaranteed your revenge killer will be able to do its job.
too suffers from magearna, so magearn I think will be a positive one, not a negative one
and 
should hopefully deal with magearn
