I don't think Tornadus is broken in this tier, but it certainly makes the tier less enjoyable by being so good. There is AV that can click knock off for free on the incoming Torn check/counter, limiting its survivability, and then u-turn and continue to do this while easily being to outlast a lot of its answers. Even staying in on your answers is usually low risk with AV, as you can either live one hit and then click knock off or pivot into a more offensive check. In some cases, even switching into your answers with Tornadus can be a good play. Boots sets with both taunt and knock can also be very annoying for some defensive checks.
Surprised the subject of whether Torn is busted doesn’t garner more discussion, from where I’m looking it really is hard to find any drawbacks to its use other than having to run inaccurate moves unless using the physical set (which is definitely a sleeper).
It’s really uniquely threatening in that it can either just take the outlast route vs some of its checks via knock+uturn+regen making stuff like rotom etc. just really poor into it, or just going from the other side and mauling everything that isn’t like Bellibolt with fly/fight coverage. I’ve tried building a lot in this tier now over the last couple of months and it just seems really asinine whatever route you take unless you run Bellibolt 24/7. Would be interested to see how others find it because I really am stumped.
Offensively, it is quite annoying to handle. Thundy-T gets outsped, knocked, and then becomes worse into Tornadus. Rotom-W isn't a threat offensively, but you can opt to run Thunder Wave to cripple it if Torn stays in on the turn you do so. Shocks is a decent check, but has no recovery and needs tbolt to threaten it enough. Arcanine-H and Oricorio-Pom Pom are both solid offensive answers since they both have access to recovery, but still tricky to play in to Torn due to their rocks weakness. Of course, Arcanine-H has to worry about focus blast/grass knot as well.
Defensively, it is more manageable. Empoleon can knock it, ice beam it, slow pivot, and roost of damage, but it has to worry about focus blast and taunt. Jirachi is an underused answer, and has everything it needs to deal with Tornadus-T. There's body slam to cripple it while bypassing taunt, wish to recover off its hits, and u-turn to slow pivot out of it. Slowking can come in and pivot out with chilly, but needs to be cautious of taunt. Gastrodon is a questionable answer due to the possibility of grass knot and taunt. Cresselia might be an option, but is not so good into Taunt and NP variants. There are other defensive answers like Goodra-H, Tinkaton, and Tyranitar, but of course they lack recovery and Goodra-H and Tyranitar fear Focus Blast.
There are also some alternative ways to mess with Tornadus as well as even get a surprise kill on it. There have been a few Stun Spore Sinistchas that have been used in tournament play, and that can be very useful to deal with Torn. That also makes me think that Stun Spore could be used on Amoonguss to also get it on the switch in. You could even click it while Torn is in if you're brave enough. Thunder Wave Cresselia, Slowking, Slowbro, or even Thundurus-I could be used to mess with it. Scarfed Mienshaos running stone edge can surprise Tornadus, but only if they are chipped as stone edge does 72-86 to the analysis recommended 208 hp AV Torn, and 85-101 to a Tornadus lacking HP investment. Thunder Punch Meowscarada can also surprise Torn, as a banded TPunch is a guaranteed kill, while non banded Meows will do 88-104. Scarf could also be used on some of its usual offensive answers to get surprised kills on unsuspecting Tornadi clicking U-Turn.
It has quite a few answers, but it is not hard for Tornadus to have a more favorable interaction than its answers. Knock off/U-Turn/Regenerator is a busted combination, and makes using Tornadus free, easy to use, and normally low risk to medium/high reward. It is hard for Tornadus to not find a use in any game with all the offensive, stall breaking, and defensive utility it can give you. Some of the counterplay I listed above is underused, and should probably be explored more with how centralizing this mon is. As far as bustedness goes, It doesn't seem OP but it is very close.