If they boost and you Tera Rock Tomb, their speed drops, making them easy to KO on the following turns. If they Tera Acro and you Tera Rock Tomb, they lose a mon. It isn't a 50/50. It's just a turn. There's a ton of options for you and them, and those options are all context-dependent. It can be favorable to you or your opponent, as can any turn.
First, off, just wanna tell you this:
+1 252 Atk Roaring Moon Acrobatics (144 BP) vs. 0 HP / 4 Def Breloom: 221-261 (84.6 - 100%) -- 6.3% chance to OHKO
So after a DD, tera acro does just blow through tera rock breloom regardless, though this is irrelevant to the discussion as a whole.
Firstly, the issue is that offensive mons abuse terastalization too bad. Yes, defensive tera might be neutered by the clause of choosing a singular tera mon, but if that balances out the mechanic as a whole offensively, why call it a bad thing? Although I would personally prefer to remove the mechanic itself, if by restricting to one tera mon we make the mechanic as a whole more bearable for many playstyles, then what is the issue?
Additionally, I just want to point to something that people mention about the metagame being new.
We started out with Tera Dark Roaring Moon. Cool mon, but it can be revenged easily by a ton of the mons in the tier, and is still walled by Great Tusk. So we got to Tera Flying Roaring Moon, which beats Great Tusk. Now we see the occasional Tera Steel and Electric Great Tusk to beat TF Roaring Moon, and as a counter-response, Tera Ground Roaring Moon. This is just the development of a singular mon in a single month. Dragonite is similarly egregious, with tons of sets available to readily deal with standard checks. If we have this much development in such a short period, what's going to happen over a couple months? Yes, it may stagnate, but Tera promotes too much diversity in teambuilding, in a way that stretches common defensive cores thin.
The main issue is this: One way we define brokenness is by looking at a mon. If it can comfortably run multiple good sets, all of which are incredibly dominant and only have a few checks, and you have to guess(or predict based on team) what it would be, the mon is probably broken to an extent, especially if you need to consider multiple mons on your team just to check different possible sets. The roaring moon and tusk scenario is one example where you never really know if the opponent is running a bait set or the set you expected, or a multi-layered bait like Tera Ground. A one mon mechanic would at least let players have a team built around a single tera wincon that could flip the game, but will not guarantee to be incredibly threatening every game.
Building is the main issue. Oftentimes it feels like you're not really building to play a 1v1, you're building to play a 1v2, since your opponents team has a whole second set of typings. You can't feasibly scout out a Tera with a uturn/switch since the timing is not necessarily that predictable (at least on high ladder, since low ladder seems to just toss it out when they start a sweep), not without losing out on significant positioning and momentum at least. Why does your team have to be able to deal with a Tera Normal Dnite as well as a Tera Fire Dnite, and on top of this potentially even a Tera Steel or Fighting Dnite. There's just way too much to account for in the builder, and games are a lot more matchup fishy as a result.
Pokemon is a game of luck, but to an extent, we players and the OU council, want to minimize that to a reasonable extent. Tera really just feels like every battle is a 6v6 where each mon is trying to bait out the other mons set, hoping that they end up having the right type matchup/movepool. It might be hard to notice but tera really just exacerbates the issues that pokemon already has underlying, just making them to an unreasonable and uncompetitive amount. Having a single tera mon is nowhere near this egregious, as single bait mons are common parts of offensive and defensive structures, and scouting for a single mons potential tera is a lot safer than scouting 6 different ones.
Tera's own offensive potential is another huge issue. Superstabs are just like Z moves, but having them last the whole game makes them much more difficult to deal with, especially in such an offensive metagame (which Gholdengo hazard stack is no doubt heavily contributing to). Knowing that one mon is going to superstab is a lot easier to deal with in the builder, though in practice a lot of these mons might just be able to break past their checks and plow through the game with superstab (tera dragon specs pult can 2hko blissey from 60%). Granted, I would much rather enter a battle knowing what gauntlet I will be put through rather than cast completely blind against a team that can't really be easily guessed from preview.
Ultimately, my choice of options would be between banning the mechanic altogether and allowing a single tera mon on your team/team preview, though I prefer banning the mechanic altogether bc superstab still remains an issue whatever option is chosen.