
Even though Scizor is a very prominent threat and bulky Ground-types like Swampert and Hippowdon and particularly Rhyperior are very good right now, Nihilego shouldn't drop from B+. It can threaten the increase in these bulky Ground-types and Water-types with Grass Knot and Z-Thunder, and it's also a very consistent Stealth Rock setter that doesn't struggle with passivity like many do. As Juuno said, the fact this can switch into Pokemon like Moltres and Mega Pidgeot, as well as a large number of other key special attackers like Mega Manectric and most Hydreigon variants too. Just because there is an increase in hazard removal, it doesn't mean at all that hazard setters should drop in viability; in fact, this can beat most Defoggers so it's useful to ensure hazards stay up in a meta full of Defoggers.
This Pokemon hasn't been great for a while, and C+ is way too generous for a Pokemon that's punished by every form of priority and the influx of faster Pokemon and U-turn users. Its lack of bulk makes it impossible to utilize its ability in the way it has the potential to be, and its offenses are good, but you don't have opportunities to set up Swords Dance at all to make it threatening. Losing out on teh ability to use Pokemon like Mega Aerodactyl and Mega Altaria costs too much for this.
Even though Rhyperior is really fulfilling its niche in a metagame where it can check Flying-types and Mega Manectric as well as a lot of physical attackers to ensure hazards stay up, I don't think it's A- material. It doesn't have good matchups against a lot of the upper ranks, like Serperior, Breloom, Suicune, and Primarina. It isn't as passive as other Stealth Rock setters, but its weaknesses to some of UU's most commmon typings does cause it to struggle in some matchups. A rise is definitely deserved because Rhyperior has much more worth than other B Pokemon like Raikou and Salazzle, and its ability to switch into a lot of threats does make it worthwhile.
I've wonderedfor a while why Venomoth was ranked the same as Pokemon like Vileplume, Feraligatr, and Florges, and I think now is the time it should drop. The meta hasn't been too kind to it, as its niche isn't needed as much. While UU may lack Quiver Dance users, other special setup sweepers like Calm Mind Latias, Nasty Plot Celebi, and even less offensive setup users like Suicune and Mega Slowbro are more more useful than this. As a Poison-type, it's outclassed by Pokemon like Nidoking and Nihilego as well as Salazzle, which can set up faster, has better base Special Attack and Speed, and can hit Steel-types super effectively. Its type coverage is lacking; even with Tinted Lens it can't break thrugh Pokemon like Cobalion and Gliscor, unless boosted. Yanmega as a Tinted Lens user offers a lot more; it can pivot with U-turn, use Giga Drain, and not have to rely on boosts or Z-Moves to perform its role. Choice Scarf users like Latias, Infernape, Nihilego, and even Hydreigon can easily revenge kill it and stop a potential sweep. Its Stealth Rock weakness also doesn't help it; it requires team support to function, and teams would much rather prefer a stronger Z-Move user with better coverage, not having to rely on only its STAB moves. Its Sleep Powder niche also offers little due to Breloom's ability to do the same (put foes to sleep then set up, but more efficiently, and the meta has adapted to sleep inducers more as a result). The meta has adapted to it, and it shouldn't be C+ any more, as the other Pokemon in that rank offer more for teams.