With no info, an unknown Sneasler set applies immense pressure even when it's not out because you always know that if you fuck up, or if they double switch at the right time, the game has a chance to be over immediately.
See, that's where I disagree. The game is not over when you send out Sneasler, even if you activate its item in the process through u-turn or from Terrain, you still get the option to switch into one of the checks I mentioned earlier and cripple it. Frankly, your statement about it being very threatening at +0 is false.
252+ Atk Sneasler Close Combat vs. 0 HP / 4 Def Great Tusk: 169-201 (45.5 - 54.1%) -- 3.9% chance to 2HKO after Leftovers recovery
252+ Atk Sneasler Close Combat vs. 0 HP / 0 Def Garchomp: 225-265 (63 - 74.2%) -- guaranteed 2HKO
252+ Atk Sneasler Close Combat vs. 0 HP / 0 Def Walking Wake: 234-276 (69 - 81.4%) -- guaranteed 2HKO
252+ Atk Sneasler Close Combat vs. 252 HP / 244+ Def Rotom-Wash: 150-177 (49.3 - 58.2%) -- 59% chance to 2HKO after Leftovers recovery
252+ Atk Sneasler Close Combat vs. 224 HP / 0 Def Cinderace: 273-322 (76.4 - 90.1%) -- guaranteed 2HKO
252+ Atk Tera Flying Sneasler Acrobatics (110 BP) vs. 0 HP / 124 Def Iron Moth: 249-294 (82.7 - 97.6%) -- guaranteed 2HKO
It gets all these 2HKOs, but the damning thing is that Unburden only activates once, so if you are forced out like literally all of these pokemon can do to you, you're now outsped by +base 105s which I don't have to tell you isn't ideal for sweeping.
Even in the best case scenario where you manage to get in on a nonthreatening move AND activate unburden at the same time, so that you are able to get a swords dance up, the game isn't over. The checks I mentioned can (mostly- Rotom isn't included) all take a hit from +2 Sneasler:
+2 252+ Atk Sneasler Gunk Shot vs. 0 HP / 4 Def Dragapult: 271-319 (85.4 - 100.6%) -- 6.3% chance to OHKO
+2 252+ Atk Sneasler Close Combat vs. 244 HP / 0 Def Gliscor: 177-209 (50.2 - 59.3%) -- 25.8% chance to 2HKO after Poison Heal
+2 252+ Atk Sneasler Close Combat vs. 252 HP / 0 Def Great Tusk: 340-402 (78.3 - 92.6%) -- guaranteed 2HKO
+2 252+ Atk Sneasler Close Combat vs. 252 HP / 252+ Def Landorus-Therian: 165-195 (43.1 - 51%) -- 5.9% chance to 2HKO (doesn't learn Ice Punch btw)
+2 252+ Atk Sneasler Close Combat vs. 0 HP / 124 Def Iron Moth: 271-319 (90 - 105.9%) -- 37.5% chance to OHKO (not great, but can be used in an emergency)
Sneasler Close Combat vs. Gholdengo: 0-0 (0 - 0%) -- possibly the worst move ever
Notice how none of these are stall or particularly defensive balance. They are all pokemon you would reasonably expect to run on offense. And again, all of these can force it out or at the very least cripple it before dying. They can force a trade.
I'd like to end the argument with the mention that surprise Teras can be effective against it, as due to its lowered defenses thanks to Close Combat, 1 wrong turn can spell its doom. Examples would be Ghost Chomp, Flying Tran, Ghost/Flying/Bug Gambit, even shit like Steel Hatt can function in the worst-case scenario.
Sure, you can argue that with rocks support and/or minor chip on the opposing team, it can achieve those OHKO, but by that point its the best case scenario where you've managed to set up both stats AND have rocks and chip up, and you successfully bait or predict around the Tera, you probably deserve to win anyway. Sneasler doesn't need to be unviable garbage to not be banworthy, and certainly not more than Ogerpon-H(--owTheFuckDoICounter), which forces Tera Dragon Dozo and Tusk as "defensive counterplay."
Oh, and also
252+ Atk Choice Band Tinted Lens Tera Dark Lokix Sucker Punch vs. 0 HP / 4 Def Sneasler: 318-376 (105.6 - 124.9%) -- guaranteed OHKO
literally unviable