If this statement holds true (which it does) then chansey should go OU as dusclops and chansey have the most shallow move pools in the game other then shuckle. Also yes 3 quiver dances volcarona can do all of that and dusclops survive but I personally think that volcarona should go UU due to stealth rock damage. Plus tran and chandulure malse that thing with no HP ground.
Dusknoir was actually considered one of the "elite 4 of suck" back in 4th gen along with Weavile, Electivire, and Ninjask (sometimes Umbreon although it only made OU twice). It actually is much less bulky than Dusclops due to Eviolite, and due to incredibly low HP, Sucknoir's bulk isn't impressive... at all. Rotom-A was a better spinblocker last gen, Jellicent is a better spinblocker this gen. There is no way in hell Dusknoir can justify a spot in OU or even UU for that matter.
Volcorona is absolutely not UU. 85/65/105 defenses are nothing to scoff at. 135 SpA and 100 Spe make it the most powerful special attacking bug in Pokemon. It also has Quiver Dance, Fiery Dance in its arsenal. Quiver Dance makes
BUTTERFREE a viable UU threat. With Volcorona's monster base stats, its no wonder its OU. Fiery Dance gives it advantages over every other fire type in the game being able to score a KO and a boost at the same time. Morning Sun and Rest can alleviate the SR problem, or hell, just use some Spin support. Regarding the Heatran/ Chandelure comment, pretty much every OU threat has a hard counter. Thundurus has Quagsire, Excadrill has Gliscor. In fact, Heatran/ Chandelure aren't even hard counters, as HP Ground mess them up badly, and HP Rock messes with others that resist Fire+Bug. The only real counter to Volcorona in existence is Jellicent.