You can't compare ho-oh to volca when considering the SR weakness. Ho-oh's stats more than make it up, which is why he even does so good in ubers. Volca's stats do not make up the stealth rock weakness; pretty much one more attack and it's dead.
And rock types ARE specially bulky in sand, which is a very common weather. Pretty much rain and sand screw volca over.
lets back up our statements with damage calcs please
Note that this is MODEST so some jolly sets end up doing even less.
252SpAtk Swarm Volcarona (+SAtk) Fire Blast in Sun vs 0HP/4SpDef Gyarados (Neutral): 49% - 58% (165 - 195 HP). Guaranteed 3HKO. 26% chance to 2HKO.
252SpAtk Swarm Volcarona (+SAtk) Fire Blast in Rain vs 0HP/4SpDef Gyarados (Neutral): 16% - 19% (55 - 66 HP). Guaranteed 7HKO.
252SpAtk +1 Swarm Volcarona (+SAtk) Fire Blast in Sun vs 0HP/4SpDef Gyarados (Neutral): 74% - 87% (247 - 291 HP). Guaranteed 2HKO.
252SpAtk +1 Swarm Volcarona (+SAtk) Fire Blast in Rain vs 0HP/4SpDef Gyarados (Neutral): 25% - 29% (83 - 98 HP). Guaranteed 4HKO.
252SpAtk +1 Swarm Volcarona (+SAtk) Fire Blast vs 0HP/4SpDef Gyarados (Neutral): 49% - 58% (165 - 195 HP). Guaranteed 3HKO. 26% chance to 2HKO.
No. +1 fire blast does NOT KO without rocks up. Though rocks are commonly up.
I didn't calc other pokemon, but compare gyardos's bulk to dragonites. It will obviously do less damage. Though it would KO salamence. Terrakion does not need a scarf btw. It can take the hit all day and just counter with stone edge. Also, who said that sun will be up 100% of the time.
Also, note that was FIRE BLAST. Fiery Dance is the more commonly used alternative, and if it isn't set up, lol it wont do anything. Gyara easily revenges with waterfall. Lets check out the other pokemon you said weren't checks.
Also, that was non-bulky gyara. Here's max HP gyarados, +1, and in Sun (its pretty common)
252SpAtk +1 Swarm Volcarona (+SAtk) Fire Blast in Sun vs 252HP/0SpDef Gyarados (Neutral): 62% - 74% (248 - 292 HP). Guaranteed 2HKO.
Consider all the other pokemon; in sand, rock types have doubled special defense. and sand is extremely common, meaning that a whole bunch of rock types can now take the attack decently.
I also like how you say that DRAGONITE is not a check to volca. Not only can dragonite end up setting up on its own with dragon dance and kill it with stone edge or earth-quake or something, but it can also phaze if it has dragon tail.
Garchomp was a league of it's own; it had sand veil. The popular sub yache set was horrendously broken, and is not comparable to volca. It could easily set up, given the common 20% miss chance, and even if you did hit with a super effective ice beam, chomp could just set up a sub and proceed to sweep.
Volcarona is simply checked by too many things. Saying things like dragonite and salamence aren't checks is just dumb. They wont end up being setup bait, what do you think they'll do, just sit there while volca sets up? no, they will counter with their own powerful attacks, which is exactly what a check does.
I'm not saying volca is a bad pokemon, its far from it. But it is simply not broken in OU standards.