Shoutout to this suspect test for sending my sanity to the shadow realm. Ladder somehow feels like the worst it's been the entire generation and that's really saying something. But now that I'm finally done, I have a lot to say on the matter of Terastilization, and why I fully intend to vote to ban it.
To start with the most foremost problem with Tera, it creates an imbalanced dynamic where offense is heavily favored over defense by virtue of the mechanic bringing far greater benefits to faster playstyles than slower ones. By giving already dangerous pokemon such as Raging Bolt, Dragonite, Kyurem or previously banned targets like Kingambit, Gholdengo and even something like Annihilape, the ability to shift away from primary type weaknesses, it allows them to approach a snowball state quicker by getting more turns, and punishes less active playstyles that can't punish them or limit them quick enough. Though it's usually possible in isolated instances to minimize a Terastilized threat, for example Tera Steel Dragonite, it usually requires very reactionary play especially if the player facing it didn't expect the move, and often involves giving up at least one pokemon or overly specific lines of play to keep it from doing more damage. And this gets more troubling when those Tera threats are backed by equally menacing, tough to handle threats that can capitalize on a weakened team. Now in isolation, one or two threats, or a handful even, wouldn't be so bad.
But there are far more Tera abusing threats that one has to account for, leading to a huge amount of stress on the builder for defensive teams trying to build consistently. I cannot tell you how many times I've tried to put something experimental together in the hopes of maybe finding something semi-consistent, going for a while thinking it works, only to run into a problem match up I simply couldn't have prepared for because there simply is not nearly enough good pokemon that can compress roles as checks to such a wide range of dangerous threats. And because of this, if playing/building Balance your guaranteed to always have some kind of holes in your teams that you just have to hope you don't run into the pokemon you weren't able to prepare for.
It can be said to a degree that pokemon like Galar Slowking or Garganacl help keep Balance working at all, as their fairly wide reach of defensive utility when factoring tera alongside their natural qualities help compress checks to a fair number of things, but it's really unrealistic for them to handle so much, and when teams have to rely on specific pokemon this much, it results in teambuilding problems where other styles can build specifically to exploit these pokemon in certain ways. Garganacl's Salt Cure is a great way to slow down and limit forms of offense as typically they don't have a ton of easy stops to it, but it can be as simple as tacking on surprise Substitute set up sweepers to take advantage of it. This happened to me more than I care for during my suspect run, as I came across all manner of random Substitute users that normally would never run it (SD Garchomp, CM Iron Crown, CM or SD Valiant, SD Landorus-T, CM+Z Tapu Lele, Belly Drum Kommo-O, and still more). This of course doesn't even factor Tera abuse on set up threats that may pull this, which makes the prospect of handling them even more ridiculously unreasonable. Unless you force yourself into super tight, rigid structures, it simply isn't happening. And even those structures are still not foolproof at all.
I've said it before, but there is a laundry list of Tera abusers, who many may not be broken by themselves but when paired up on the same team can result in cores that easily overwhelm many defensive teams. Iron Crown, Raging Bolt, Archaludon, Dragonite, Kyurem, Ogerpon-Wellspring, Iron Valiant, Iron Moth, Tapu Lele, Volcarona, Urshifu-R, those are the main ones that come to mind. Now consider trying to account for various combinations of these threats used together on teams, in addition to other non Tera threats like Mega Medicham, ZardY, Mega Diancie, Z Lando, and you'll quickly see how outlandish the idea is. Then if you somehow do have a somewhat okay defensive team, then you realize in battle that your check to something is only a check so long as you're facing the Tera type of a threat you prepared for.
I'd like to briefly touch on a few claims made by some posters, that the fault of this metagame's issues doesn't like with Tera, but rather the removal of certain pokemon that helped keep a few things in line and offered some good aspects to teambuilding (the main pokemon to mind being Zamazenta but there were others I'm sure some people are referring to). It's really easy to make the claim in hindsight that maybe it wasn't the best idea to remove certain pokemon by banning them, but we were tiering at those times, we tiered with that period of the metagame in mind. We don't tier with thoughts of what may happen as a result of a ban, because frankly we cannot realistically know just how much a metagame will change as a result. Zamazenta's ban might be argued as pushing Darkrai over, but that was borderline broken when the dog was around anyways. Some might say removing Gholdengo was a mistake as it did bring some good defensive traits to teambuilding, but it also had a similarly toxic effect on defensive teams with its Bulky NP sets that, with Tera, were very hard to punish and limit. We removed these issues because in those moments, because they felt like they needed to go.
I ran bulky teams anyways despite these problems not because I thought it was a good idea (spoilers, it wasn't), but because I wanted to prove a point. It was miserable beyond belief, and it felt like every other game there was some Tera abuse come up that I didn't prep for, and couldn't possibly have. My run was not remotely clean at all, it's pretty ugly. And it was made worse by the uptick in stall to combat the HO running everywhere, which is a playstyle I also couldn't prep for. It's pretty alarming, and a major warning sign of a serious problem when other bulkier teams are almost nonexistent and players have resorted to Stall just to try and be somewhat consistent. I actually did talk with several players while getting reqs, and most of them agreed how foolish it was to try and run defense that wasn't Stall on this ladder.
Tera overwhelmingly favors offensive playstyles, and the use of this mechanic is greatly weighted towards their success. It's much more efficient to use for an offensive threat whether setting up and sweeping or using a hyper strong wallbreaker to truly lock down defense, rather than try and react with Tera on a defensive team. Its presence has majorly helped push the viability of Balance to near unviability (and at this point I'm wondering if it straight up is just unviable), and it's caused a massive stagnation in building and playing. We've tried over and over with suspects of individual pokemon, removing problems hoping to improve the tier, but it has not.
Will removing Terastilization solve EVERY issue this tier has? Doubtful, even as a staunchly pro ban player, but it's a crucial step in the right direction to reduce volatility, reduce variance and threat saturation, and improve consistency with teambuilding. I will strongly be voting BAN Tera.
Dragonite can reasonably run Tera Steel (on 2A Dragon Dance sets), Tera Fire (on DD + Scale Shot + Fire Punch), or the rarer Tera Normal (on like the random Curse sets). 
Garganacl can reasonably run Tera Water or Tera Fairy, each offering their separate defensive benefits. 
Valiant can reasonably run Tera Dark on Swords Dance sets, Tera Electric on Calm Mind sets, and Tera Fairy on Specs sets (I've experimented with Tera Ghost, but haven't seen it prevalently). 
Volcarona can run either Tera Grass on its offensive set or Tera Steel on its bulky set. 
