Imma quote the post where I said which are, imo, all viable. I'll also add to the list tera water Glowking thunder waving/ Toxic and then pivoting to a mon that will more easily deal with the crippled Darkrai.
It's rather easy to exploit the forced use of Tera by the rest of Darkrai's team which is why i've never been a fan of this kind of argument, since it often puts the opponent in a lose of resources.
Before I touch on the next bit, I wanna address the offensive counterplay you listed before. Gambit/Dragonite is good, but it does struggle with Wisp variants as Darkrai being able to cripple both goes a long way to aiding its teammates to sweep later as they're threatened less by the burned prio users. Vacuum Wave Valiant... how many people are using that? Val already has crowded moveslots on every single set and I can't think of a reason you'd ever want it over the many better options. Rilla can only really revenge kill and Rilla actually does struggle a bit into certain Darkrai structures that tend to pack fat offensive resists to its grassy glide which makes it not as useful as you'd think. Banded Scizor is fine, if a bit hard to fit sometimes but it's a good answer I'll give you that. Specs Raging Bolt though... can't comment on how practical that actually is since I haven't seen it used in higher level play but I'm personally not a fan of it in concept because it's dependance on a priority move that fails if the opponen doesn't attack actually can be really exploitable, especially by protect users which are common on Balance.
For the record, I appreciate you bringing these examples. I think it's helpful and adds to discussion a lot. That said, I do have issues with some of them mainly because they're rather circumstantial answers. Obviously Kingambit may check Darkrai without Wisp/FB, or Dragonite/Gambit can help vs it without Wisp or FB+Ice Beam, but it's difficult to gleam what it's running from the outset of battle and potentially guessing wrong can give the Darkrai user a massive advantage. This goes for Zama and Tusk too which get ruined by wisp variants (and knock off cripples Zama). This is all disregarding the threat of Specs which has such immediate power that you'd want to respect its possibility quite highly. Switching in Zama expecting nasty plot only to eat a specs sludge bomb is nasty.
As for the defensive answers, the thing is you can also make that argument about Waterpon, Kyurem, Roaring Moon (curiously the four of them are together on the usage stats), and unironically Raging Bolt. So while this is true for Darkrai, is also true for basically any breaker in the tier.
Darkrai can slap utility yeah and that means it has less coverage to hit all its checks. It can't hit everything it wants with 2 or even 3 damaging moves and I think that balances it. The fact that Darkrai can cripple its answers can, again, be said for Kyurem with freeze, Raging Bolt with para etctera.
Is it good? For sure, really good I'd say.
Do I find it broken? No, imo it's not broken.
Now then, I don't disagree about Wellspring or Kyurem necessarily (Roaring Moon actually has fairly adequate counterplay right now between priority, chip and contact punishing, sticky barb clef and other strategies), the difference is that these pokemon aren't as volatile in their set possibilities which makes prepping for them more straightforward. I'd love to axe Kyurem too, and don't care for Wellspring, but I think But Darkrai's versatility and how punishing guessing the wrong set can be is, I believe, too much.
Not sure what Raging Bolt is listed for though. Undeniably great, but it's also honestly the most overhyped pokemon in terms of presence and impact. It rarely ever sweeps because it's something fairly straightforward to prep for and it's much more prone to chip due to its low speed and often being drawn in to defensively check things, which results in it being fairly easy to limit longterm. Again, great mon but I don't think it's anywhere remotely close to the big three balance breakers, let alone problematic.
Gliscor is not as powerful as in DLC1, I agree. But that does not mean it's really hard to face and most of the arguments made for a lot of Pokémon (not a lot of answers for most all of its sets, a lot of utility inside its kit, contrains teambuilder...) can also be argued for Gliscor. You're a Toxic away from basically losing your breaker if it's defensive, and since SD sets usually also pack Protect, it can actually turn the tables and get a big hit on a defensive pivot thinking it can safely switch in. Gliscor (as Garg btw) can by using its strong residual move (Gliscor is toxic, Garg is Salt cure) cripple an offensive mon and most probably win it 1v1. I found their easy way to punish almost every mon in almost every team structure (asides from Clefable ig, which I actually found quite meh if not for this two matchups) not really healthy. This is just my point of view tbh.
For example, you mention Ting-Lu and I actually find it a really cool mon in the tier with its clear limitation in not any recovery aside from Leftovers for example, and not being able to "sweep" or easily clean games since it doesn't have a boosting move.
I may be too biased aganist Gliscor tho... But I tried to make a solid point ^^" but I could be wrong ofc, I always like to debate about this stuff:)
Gliscor's great, no denying it, but there are match ups where the aggressive pace of some teams don't give it as many opportunities to be annoying and it especially dislikes the presence of Kyurem and Wellspring, the latter especially punishing SD sets pretty hard. Phys Def Glowking with Ice Beam can snipe Gliscor fairly safely as well, and Darkrai of course has that option as well. Gliscor also can't handle already boosted threats as easily, and without knock off it actually gives free turns to Balloon Gholdengo. it's important to identify what will give opposing Gliscor chances to come in so you can plan around these instances and sometimes, abuse them. Heck even Garg is annoying for non knock Gliscor since SR+Salt Cure means it's not geting any health back and boosting Garg can just beat Gliscor.
I totally get when a mon is frustrating though. No need to feel bad about a bias. I have my own. We all do! And debate like this is a lot more healthy and productive. So thanks for adding your input.