Reasoning

Blissey has been warping the metagame around itself to an extent for quite some time now. It's not an understatement when I claim that the viability of every single set of every single special attacker is influenced by the extent of progress it can make against Blissey. Take a look at Ash-Greninja and Nasty Plot Hydreigon; once two of the most formidable metagame-presences, they have now been reduced to nothing but a wisp of their former selves, solely because they lack the means to make meaningful progress versus Blissey. Special attackers that háve established themselves as good Pokemon, all have done so partly due to their ability to combat Blissey: Mega Latias has Psyshock; Magearna can Volt Switch out, exploit Blissey with Pain Split, and neutralize Seismic Toss with a boosted Draining Kiss; Heatran can overwhelm Blissey with the combination of Magma Storm, Toxic, and Taunt; and Clefable sports the necessary resiliency to continually boost up against Blissey and overwhelm it eventually.
Beyond this sheer influence, Blissey is also a tremendous enabler for many wallbreakers that otherwise have a hard time finding opportunities to switch in, courtesy of its huge special bulk and Teleport. There really is no other Pokemon that provides nearly as many opportunities for its teammates as Blissey is able to. Add Heavy-Duty Boots and Natural Cure to the equation, and you are looking at a Pokemon that is beyond a doubt the most effective, influential, and hard-to-punish enabler in National Dex. It has gotten to a point where forgoing Blissey on a bulky team feels like a genuine opportunity cost, a clear sign that it might be executing its job a bit too effectively for its own good.

Calm Mind Mega Latias is a tremendous wincondition. It lacks a true Achilles heel other than a weakness to Toxic, between its ginormous defenses, access to setup and recovery, great Special Attack, a good Speed tier, and a wide repertoire of coverage options. Mega Latias's most common set, Psyshock + Aura Sphere, is a really hard-to-contain set in itself. Make no mistake, this set does have a pretty sufficient amount of checks on paper. Examples include a healthy Mega Mawile or Crunch (Mega) Tyranitar, Mega Scizor, the combination of Corviknight / Slowbro / Blissey + Weavile / Ghostium Z Spectrier, Heart Swap Magearna, and the stray Slowbro, Blissey, and Clefable running Toxic. The issue, however, stems from Mega Latias's ability to overcome most of these checks on its own in practice with its sheer staying-prowess. Mega Tyranitar fails to stand up to a healthy Mega Latias at +1. Weavile has to risk the unpredictable Triple Axel to stand a realistic chance of revenge killing Mega Latias and will be taken out by Aura Sphere, should it fail to connect all three hits. The OHKO from Spectrier's Never-Ending Nighmare is avoided after just one Calm Mind boost. Heart Swap Magearna has to wait until Mega Latias has accrued multiple boosts, else it will be dragged into a sequence of using Heart Swap against a continuously boosting Mega Latias, a sequence it is doomed to lose because of Psyshock.
To add, the flexibility Mega Latias can afford is definitely noteworthy as well; while Psyshock + Aura Sphere is the common set, Thunderbolt + Ice Beam and Refresh variants should not be overlooked. Both set variations add depth to Mega Latias as a whole, with Thunderbolt + Ice Beam overwhelming traditional checks in Slowbro and Corviknight, and Refresh removing Mega Latias's weakness to the aforementioned stray Toxic users. All things considered, it's not hard to imagine why Mega Latias would contribute to a constricted teambuilder and matchup issues.

Magearna lacks counters, period, and it establishes this statement with merely a Choice Specs attached to it. Choice Specs Magearna boasts an insanely high ceiling in the right hands, since it faces a grand total of one insurmountable matchup in the extremely niche Shedinja. It's capable of overwhelming its traditional checks like Blissey, Mega Scizor, and Aegislash by continuously chipping them down with a strong Volt Switch -especially with entry hazards and Future Sight in the full context- before sealing the 2HKO with either Fleur Cannon or Focus Blast / Aura Sphere. Magearna also has the luxury of possessing over an unblock-able Volt Switch; even the sturdiest Volt Switch-blockers in Gliscor and Gastrodon are cleanly 2HKOed by Fleur Cannon without any prior chip damage required. Fighting-coverage further jeopardizes your chances of scouting Choice Specs Magearna properly, swiftly taking down the likes of Heatran, Excadrill, and Ferrothorn. Trick is the cherry on top, crippling just about any defensive Pokemon, notably including Amoonguss and Galarian Slowking, the only Pokemon that could have somewhat maintained a neutral momentum trade against Magearna otherwise.
Beyond Choice Specs, we have also seen Double Dance Kisspower rising to stardom in the past few weeks. In retrospect, this isn't surprising; Kisspower Magearna also has a painful lack of true checks, with Heatran, Excadrill, and Gyro Ball Ferrothorn being some of the only ones. The combination of these two sets allows Magearna to really test the limits of this metagame and its teambuilder. Choice Specs really only can be contained by meticulous positioning and offensive pressure, which Kisspower benefits from due to how it uses pivots to Choice Specs such as Blissey, Toxapex, and Amoonguss as setup bait. Conversely, Kisspower has a minimal amount of checks as is, all of which crumble to Choice Specs Focus Blast.

It may not get as many opportunities as Blissey does, but Slowbro remains a disgustingly effective enabler with Teleport that distinguishes itself with Future Sight and Regenerator. This combination allows Slowbro to set up very favorable situations for its teammates that are sometimes nigh-inescapable for the opponent, all the while letting it preserve momentum well courtesy of Regenerator and Heavy-Duty Boots. For the sake of some examples, let's analyze a situation where Slowbro sets up Future Sight and Teleports into Mega Lopunny, Weavile, or Mega Charizard Y. The opponent, being a competent builder, has obviously brought a check to these Pokemon in Toxapex. Regardless, they now find themselves forced into a checkmate position; they either send out Toxapex, which is bound to falter to Future Sight + any combination of two attacks the aforementioned Pokemon can dish out, or they find themselves hard-pressed to sack another Pokemon in their attempt to preserve Toxapex. The same dynamic can be observed in many more situations: the likes of Mega Medicham, Garchomp, and Kyurem find themselves breaking through Clefable, and the likes of Kartana, Mega Lopunny, and Excadrill face no issues in making progress against hard-checks like Zapdos or Tangrowth. This interaction obviously adds up to the strain on teambuilding, invalidating otherwise perfectly fine checks and even counters to tons of offensive Pokemon in an overall pretty foolproof way.
In short, Slowbro's combination of Future Sight + Teleport + Regenerator ensues inevitable progress-making, almost being reminiscent of infamously broken elements in Shadow Tag and Arena Trap. It's not hard to imagine why this interaction would be regarded as unhealthy enough to put Slowbro on the radar for now.

The topic of Spectrier's position in the metagame has been beaten to death ever since it was discovered after Dragapult's ban. This is not surprising; slowly but surely, Spectrier has indeed been showing off unhealthy qualities. Don't let its seemingly deserted movepool fool you. Spectrier has plenty of tools to run through teams in full gallop between Nasty Plot, Grim Neigh, great Speed and Special Attack, STAB-moves that know a mere handful amount of resistances, and access to Z-Moves to shatter most of these resistances. There are no splashable checks to Spectrier outside of Mega Tyranitar, its base form, Mega Lopunny, and bulky Snarl Hydreigon. As a result, bulky-inclined teams are now forced to equip one of these four in order to not auto-lose to Spectrier, which puts massive strain on diverse teambuilding. Even in the face of its checks, Spectrier has the means to make an impact. Mega Tyranitar and Tyranitar need to stay fairly healthy to take on a boosted Never-Ending Nightmare, and detest Will-O-Wisp, especially if Spectrier is paired with Pokemon like Mega Latias or Heatran. Similarly, Mega Lopunny doesn't appreciate Will-O-Wisp and can only hold up for so long with its pretty measly defenses. Hydreigon needs to stay in pristine condition to come out on top versus a healthy Spectrier that uses Nasty Plot after forcing a switch and goes for another Nasty Plot before living the first Snarl: the subsequent +3 Never-Ending Nightmare has a chance to take Hydreigon out, should it be anywhere under 98% of its total health.
Simply put, Spectrier not only strangles the teambuilder as a result of a painful lack in counterplay, it forces a huge element of matchup in the metagame, ruthlessly punishes teams without counterplay, and is even capable of coming out on top against the Pokemon that dó check it with patient execution, after which it almost inevitably seals the match with bonus Grim Neigh boosts and its excellent Speed tier. As a result, it convincingly lands itself a spot in this discussion.