No, it still fears priority from Choice Band Scizor just like Salamence does. That's, of course, the problem. Furthermore, with Leftovers you now nerf the chances of getting two Dragon Dances from Scarf Garchomp and company, which would've been a big help since even +1 Ononokusu is still pretty meh in Speed and damage. It's good, but you're overrating how good it is.
The reason Haban berry is more useful then anything is because of the scarfed dragons that'll be running around in the beginning. Bulky Ono sets will survive a Draco Meteor from Latias with SR not on the field, and can stop chomp from doing ridiculous damage as well. Sazando's Draco meteor can be damped as well. Even though that Sazando, and Latias will obviously work better revenging it when not at the +2 DD marker is because they're hitting on his none defensive side (which, not even Yachi can do anything about when it comes to Ice attacks).
Im not sure what you mean about the Gyarados thing since even if Gyara got a boost to his special attack, any Ono not running with some HP stats only take around 55-59% with Yachi, and take even less from Hydro Pump. Unless you meant stone Edge? Because Stone Edge would be the only extremely damaging thing that would come from Gyara after a +1 Atk Boost, which bulky sets can live through and K.O back.
False false false. First off, we discussed this pretty extensively in #stark, where a user brought up that it was OHKOed by Scarf Latias. I even asked for the calculation, and we both confirmed that it is 100% with Stealth Rock, and pretty high chances without. For those wondering at home, the damage output with Draco Meteor is 93.2% - 109.9%. That's
with Haban Berry. Max HP takes 73.6% - 87.1%, but now you trade Attack or Speed. The good news is that Ononokusu can afford to use more EVs into HP at least since 303 is the most Speed it needs, which can be reached with Adamant. The bad news? Well, after a Draco Meteor you're not left with much breathing space. Max damage is still... 99% health gone from Stealth Rock. Then you must make sure Sandstorm isn't present. That's a big problem.
I also just proved that Yache
can help Ononokusu versus Ice-type attacks, with Starmie and +1 Ice Fang from Gyarados. It fails versus Choice Band Mamoswine... and that's about it. Haban / Yache Berry Ononokusu can work, but it requires some frightening situations. You also can't afford much bulk onto Ononokusu: Ono needs 303 Speed to outspeed Mole after two Dragon Dances. This heavily depends on what nature you use as well, since Adamant will require more Attack EVs onto it.
I haven't looked into the SD sets much, mostly because I dont like to use SD, but DD sets are still functional in various cases with Ono.
But they're simply not safe enough. Dragon Dance sets are viable, yes, then again there is too much fluctuation with Speed (Choice Scarf Latias is the least of your problems here). It's not weak to priority aside from Ice Shard, thankfully, but it's common enough to make the difference.
My tl;dr version is this: If you're using Ononokusu as a Dragon Dancer, you better have your Yache / Haban berries ready. And make sure to stay the hell away from Scarf Lati@s, which are solid checks against it.
Aside from that, Ononokusu is a threat. A decent one, but I think you're slightly overhyping its Dragon Dance "capabilities". It's a great Pokemon, for sure, since Dragon Dance
can work with resist berries and maybe removal of Stealth Rock / Sandstorm (of course that is easier said than done). I'm still more intrigued with Swords Dance Ononokusu with Taunt. Adamant Outrage steamrolls through Pokemon like nothing. For fucks sake,
the current Hippowdon EV spread gets OHKOed by +2 Adamant Outrage. Max HP / max Def Hippo takes 93.6% - 110.2%, which in other words: high ass chances to OHKO regardless so long as Stealth Rock is in play. All you really have to do is clear Skarmory and find a good Pokemon to set up on with Taunt + Swords Dance, or just Swords Dance on the switch and Taunt if it's Skarmory. I'd go as far to say that Ononokusu is the ultimate stall breaker with just Swords Dance + Outrage alone.