Dark types are still affected by Z-Hypnosis, they are only immune to prankster moves.I disagree for a couple reasons.
1. 4x rocks weakness. This cannot be stressed enough. You need reliable hazard control, which is pretty difficult in this meta since you essentially only have 4 options (M Scizor, Zapdos, Fini, Excadrill) and none are super ideal.
2. Coverage. Volcarona obviously has to run a fire type move and has to run QD, which means it only has two slots for coverage. It can run STAB Bug Buzz, or Giga Drain, or psychic, or HP Ground, but only two when it'd like all of them. Running the Shattered psyche means you're guaranteed psychic, which will probably be useless after you use the z (more than one poison type on a team is uncommon).
3. Its bulky set can't check Megagross. Its defense is ass, and even with 252 HP/252 Def it gets 2HKOed by Megagross's Zen Headbutt - which almost all of them run, despite what you said. It's pretty underwhelming in general, because 85/65/105 defenses aren't that great by any means.
4. Rise of 100+ scarfers. Nihilego, Terrakion, even stone edge Keldeo are all popular choices nowadays and are all tailor made to counter Volcarona. With them running around, Volcarona finds it much more difficult to sweep.
It's still the most devastating special set up sweeper in the tier (bar QD Phero but rip), but 4x rocks, Scarfers, and its lack of diversity hold it back from A+.
It also feels wrong for it to be two subranks above CharY when it's essentially the sweeper to Char's wallbreaker. One rank makes sense because Char takes the mega slot, but two is a bit much.
Speaking of special sweepers;
What do people think of Z Hypnosis Xurkitree? Even at +1, base 83 speed isn't the best, but having a 100% accurate sleep move (with only dark types immune to it) that grants +1 speed is pretty devastating. It basically means you have a free turn of set up, which means tail glow, and then it's off to the races. If you don't have an answer to it (like scarf terrakion), then you could be 6-0ed right then and there.
And as for the Xurkitree set, yes it can be very effective if your opponent isn't prepared for it, but aren't there a plethora of scarfers in the teir that can wreck it because it has very little in terms of defences?
EDIT: Also, can you even call it a "sweeper"? It has such piss poor speed that I really don't imagine it sweeping any sort of well built team any time soon.