I think doubles is an extremely balanced type of format in general--I think most things are relatively manageable, and bans are rare. Shadow tag has hung around for a while, and hasn't been particularly broken, but a little bit over-punishing. In an ideal world, Shadow Tag wouldn't be in the format, but I do not think it is particularly urgent. Shadow tag admits of consistent counterplay in mons like Incineroar and Dragapult, each of which hit Gothitelle super-effectively and can switch out of tag. Basically: shadow tag is good, but not cancer. It might be too good, but I think that that's by degrees.

Beat up, on the other hand, is cancer. In particular, I offer 3 claims.



1) Beat up can immediately win games in a (largely) undeserving manner. Currently the power level and set of board positions is such that it's not amazingly simple for gothitelle to come in. However, if you simply kill Whimsicott's partner as it sets up tailwind, or even kill both whimsicott and its partner, Terrakion and/or Dragapult can come in, and instantly win the game with dynamax and beat up. Getting to +6 or +5 (or even +4) can be enough to instantly win the game, so it puts a lot of pressure on the beat up counters, e.g., follow me, to be on the field constantly, and any instance of misplay guarantees loss. Basically: it is too punishing to mess up, which was the problem with e.g. goth lax last generation. That gives too much of an advantage to the beat up user.



2) Beat Up removes interactivity. While I agree with Marilli that hard TR can defeat opposing teams, I claim that it is relatively simple to outplay the hard TR player with a number of teams that are not necessarily suited to the task--i.e., don't have specific hard tr counters but just have good pokemon. The first TR always goes up, but there are ways to prevent the second tr from going up by outplaying your opponent. In other words, hard TR admits of interaction, even once it has secured the first step in the game.

Beat up does not. Once the terrakion is at +6, the game is over. it Dynamaxes, and boosts its speed and special defense and ohko's everything with rock slide. There is nothing to do once the gimmick has been achieved. Even with things like shedinja, you can pp stall and so forth. With terrakion, there's nothing to do.



3) The mirror is turbo cancer. Did you see the Kiichi vs Kaori game? Random ally switch, max air stream, random thunder wave, and then the game was over on turn 3. Terrakion just clicked rock slide every other turn.



I personally have not played too much against it because I've been gentlemanning it against Qsns, Stax and Arcticblast, my SPL and ssnls opponents respectively. when I have played against it it has felt decently strong in power level, (not unbelievably broken, my teams often didn't let my opps set it up). However I think the real problem is that the opportunity cost to run it is so low and the cancer involved by having it is so high that it's not worth it.

Further, the metagame shaping effects are severe. Every team basically has to run blastoise, or weezing, or be hyper offense to deal with Terrakion. I think that's a large restriction on teambuilding that needn't exist.