Sure, thoughts.
Rock Polish
One thing that makes the Rock Polish set so devastating is the extensive selection of viable moves it has at its disposal. While other pokemon can often run a pretty wide variety of sets, once players have established that, for example, an opponent’s Salamence is a Dragon Dance variant, they generally know what attacks to expect and can react accordingly. Genesect, however, can stock up on any combination of Ice Beam, Thunder(/bolt), Flamethrower, Giga Drain, Bug Buzz, and apparently HP Water now. Even though you can often intuit that the opponent’s Genesect is a Rock Polish build part-way through the match, you still have no idea what you can safely answer it with because nearly every potential RP Genesect check is entirely dismantled by one of the listed moves. (Terrakion: Giga Drain/HP Water, Heatran: HP Water, Rotom-W: Giga Drain, Tentacruel: Thunder, Jellicent: Thunder/Giga Drain, Tyranitar: Bug Buzz… Heatran, Specially Defensive Ninetales, and Blissey/Chansey are probably the safest answers, but these are all either cleanly picked off by Dugtrio/worn down from lack of reliable recovery or at a severe disadvantage against most of the surrounding metagame.) Running specific attacks to disrupt traditional checks isn’t a new concept, but decisions to do so often greatly limit overall coverage for the sake of eliminating such a select obstacle. Genesect, though, retains great coverage and power with almost any arrangement of the attacks listed. As a result, players are forced to carry multiple “conditional” Genesect checks to stop every moveset strand of the Rock Polish set. It might not be able to kill absolutely everything in the metagame with only three moves at its disposal, but it can kill a good majority, and by the time you figure out which hazardous moves it isn’t carrying, it’s likely that Genesect will have already done irreparable damage. The potential turn-free Special Attack boost from Download just worsens matters by letting an already potent offensive threat boost its firepower and double its speed in a time span other pokemon would kill to replicate.
Choice Scarf
I think the Scarf set actually teeters on the cusp of brokenness as well for a number of reasons already discussed in this thread. As a balanced player who hasn’t yet jumped aboard the crazy offense train, I ended up running two Rocky Helmet pokemon on my team last round just to ensure Scarf Genesect actually lost *something* when it came in. Genesect is such a disgustingly effective user of Choice Scarf because its momentum-stealing STAB U-turn is accompanied by a broad range of similarly powerful supplementary moves with incredible super effective coverage against the majority of the tier. In conjunction with one another, these factors place most defending players in a bad position picking between the lesser of multiple evils. (Genesect can U-turn for momentum if you switch and can either KO your active pokemon with a coverage move or beat it down while grabbing another advantageous matchup through U-turn if you decide to stay in.) Other comparable momentum pokemon like Scarf Landorus, Scarf Jirachi, and CB Scizor all fall short when compared with Genesect. Landorus and Jirachi have problems with, among other things, weak U-turns (which means that the defending pokemon isn’t really punished for staying in on such an attack--not the case with Genesect), while Scizor lacks the speed and coverage necessary to immediately threaten the majority of the metagame with KOs and force players into the lose-lose situation Scarf Genesect often creates. Genesect’s durable typing, impressive attacking stats, excellent movepool, and auto-activating ability render its Scarf set significantly more threatening to the metagame than the competitors previously mentioned.
I agree that the metagame contains compound strategies that are arguably more broken than our current suspect, but I don’t think it contains any other individual pokemon as multi-dimensionally dangerous as Genesect. 50% usage is unprecedented and absurd--that alone should attest to Genesect's abilities in the hands of good players.
