Venomoth can't do a single thing to Cryo. With QD it, and anything it passes to NEEDS several boosts to do anything significant to Cryo so after a worst case scenario your facing a +2 Speed Physical attacker that threatens you out as special attackers can't touch it.
Same for Yanmega. Its strongest attack against Cryo that I've seen did around 30% after Lefties.
25% from SR plus 30% from bug buzz after lefties leaves you at 45%, meaning that if it's taken more than 10% damage (which considering the fact that most teams rely on it for spinning is fairly likely) then you no longer can switch into it. On paper it checks it but in reality it's not the completely reliable failsafe counter you're making it out to be. I will post calcs later but Venomoth can handle Cryogonal by sleeping it and then spamming bug buzz (see mantine, it's basically the same process).
The three ShellSmashers admittedly aren't handled great by Cryo but it does do a fairly decent job of stopping them, Gorebyss, as you said is stopped pretty hard by it with Haze.
We're in agreement that Cryogonal can't check the shell smashers. Fried_Rhys did a good job here explaining why.
Also, by dint of testing, Mantine is such a hard counter to Yanmega it isn't funny. I'm currenty running Scald, Ice Beam, Toxic, Haze and it works pretty brilliantly as a full stop to Mega, Venemoth, and other threats to my team like BU Gallade.
Mantine is one of the mons like munchlax and regice that can switch into Yanmega once and then dies the next time it tries to switch in (you have nothing to stop it from switching out). After SR, it's down to 75%. It isn't 2hkoed after SR, but it is 2hkoed the next time you try to switch in. In fact, specs bug buzz can 3hko you and all you can manage back is a toxic or an uninvested ice beam, so you don't even counter Yanmega; it wins. It still gets its kill per game, and it's not even crippled by Mantine.I haven't seen any Mantines, so this is what I had written based on theorymon before I ran calcs. After running calcs, I came to the conclusion that it's not even that good. Modest specsmega does 38.3% - 45.2% to max/max calm mantine with bug buzz. After SR, that's a 2hko roughly 66% of the time, meaning that you don't even move most of the time. Again for emphasis, this is a max/max+ mantine.
As for Venomoth, it can sleep you and spam neutral stab or just pass out to an electric type and boost later. You can't toxic it because of its poison typing, so you're reduced to hazing it and hoping for scald burns. Cromoth destroys you.Again, using Max/Max+ Mantine, +1 leftovers 252 modest venomoth does 32.3%-38% with bug buzz. Assuming you haze the first turn after you switch in and that it doesn't go for the 75% autowin with sleep powder, it wins -even if- you get a scald burn on your first possible chance. If it does go for the sleep, it can boost again, do 43.1% - 50.6% with a +2 buzz, and you have to hope that you wake up. Even if you do wake up and haze, it can just sleep you again and get +1 before killing you. If it's life orb Venomoth it can do 41.9% - 49.4% (43.1%-50.9% with HP rock) at +1. Lum Venomoth has the same calcs as lefties but can avoid the first time you get a burn with scald (scald burns are not guaranteed anyways). Cromoth obviously destroys Mantine. Long story short,
every Venomoth set destroys Mantine. Mantine 100% loses to Venomoth.
Mantine works well for Omastar and Gorebyss, as long as niether run hidden power electric (which is more viable now that gastrodon is ou).+2 LO Omastar does 46.1% - 54.5% to Mantine with Ice Beam, which isn't enough to ever 2hko assuming a haze. However, it's a guaranteed 3hko, and all Mantine can do back is Scald for 34% - 40.4%, which is better than nothing, but you still lose. Ancientpower Omastar does 86.8% - 103% at +2, hp electric always ohkoes. Modest Life Orb +2 Gorebyss does 45.5% - 53.9% with Ice Beam, and White Herb does 35.3% - 41.3%. Like Omastar, you can stop its boosts but you still lose if it decides to attack. And same as before, HP electric ohkoes.
Huntail does 108.4% - 127.5% with LO Return, and 83.2% - 98.2% with White Herb. No chance, always an ohko after rocks.
tl;dr Mantine blocks Gorebyss from passing but dies in the process. It loses to Omastar but stops it from sweeping, barring HP electric or ancientpower. Venomoth winds up with +1 and all sets beat mantine easily. Yanmega outright 2hkoes it 66% of the time, and can stay in to ko it the rest of the time. Huntail always ohkoes it after a boost.
The best mantine can do to any of the suspects is block them, but it has to sacrifice itself to have a chance of taking down a smashed mon. It can be bypassed by changing up your moveset, and you only lose coverage on Lanturn by running HP electric over grass.
Mantine does not "hard counter" any of the suspects. Please back up your statements when posting.