Expanding upon this, Reuniclus is very bulky on the physical side, and can easily Recover off damage faster than most Pokemon can deal it. Max HP / Max Def, the standard for the Calm Mind set, can survive a U-Turn from an Adamant Choice Band Scizor (which is huge) and easily take a Crunch from most variants of Tyranitar, although Band sets can scare Reuniclus away.
It's also quite powerful after a Calm Mind, and gains significant Special bulk. Magic Guard gives it an immunity to stuff like Leech Seed, Toxic and Spikes, which means that you must take it out with direct attacks. Stall teams usually lack powerful attacks, and they can't wear Reuniclus down with passive damage, making it extremely hard for them to deal with.
After a few Calm Minds, Reuniclus is extremely bulky on both sides, and most Pokemon will fail to OHKO it, but are KOed in return. It can easily set up on an absurdly high number of Pokemon, who pose no threat to it whatsoever.
Scizor and Tyranitar are the bane of almost all Psychics, but Reuniclus can beat both of them with its standard set. It doesn't need to overstretch itself to deal with checks, it beats 80% of the metagame with one single set. This is what makes Reuniclus a top tier Pokemon.
Yeah, I agree with most of what you said, but I think stall isn't nearly as hard pressed to deal with Reuniclus as in earlier rounds when people were like switching Blissey into bulky CM. There are actually several strategies stall has adapted to play around Reuniclus that don't force it to apply overwhelming attacks. For instance, Perish Song Celebi can put Reuniclus on a three turn timer, momentarily stall its attacks out with Recover, and then U-turn to gain switch advantage if Reuniclus isn't the last mon.
Another interesting combo I saw on user Blightbringer's hail stall team (made the finals of a tour iirc) was Taunt + Night Shade/Seismic Toss on Deoxys-D. Taunt prevents Reuniclus from recovering or accumulating too many boosts, while Deoxys shakes off the resisted attacks and essentially turns Reun into spike fodder until you're ready to chip him to death with the auto-100 damage move of choice. Though, you have to be wary of SpDef drops and Deoxys does struggle against the occasional CM Reuniclus with Shadow Ball over its STAB.
Then there is Unaware Quagsire who can ignore Reuniclus' boosts, while still being a relevant counter to a lot of common bulky stat-uppers in the current metagame. I find a specially defensive Curse set is Quaggy's most reliable way of defeating CM Reuniclus so that he can comfortably set up on any attack and eventually 2HKO, rather than resorting to a PP war and taking your chances on a crit or SpDef drop.
Encore also completely owns Reuniclus because he's so slow...
On a different note, a lot of stall teams are somewhat offensively tinged these days to combat Reuniclus with things like CBTar popping up to threaten with a powerful STAB Crunch and Pursuit trap. I also like to use this ChopleTar set on one of my sand stall teams because it makes a good blend between utility and Reuniclus check (he also helped check Thundurus last round, but I used Rock Slide then over Fire Blast):
Tyranitar @ Chople Berry
EVs: 252 HP / 88 Atk / 168 SpDef
Brave Nature (+Atk, -Spd)
- Stealth Rock
- Crunch
- Pursuit
- Fire Blast
The given EVs ensure that you 2HKO Max HP/Max Def Reuniclus with Crunch and the rest go towards maximizing special bulk. Fire Blast deters opposing stall from stacking layers and a Brave nature is used so that he can maintain the 2HKO on Max HP/40 SpDef Skarm. Specially defensive TTar is also a good check for SubCM Latias and Psyshock Latios which normally give Blissey problems.
