With a very impressive base speed stat of 95, it outspeeds many of the offensive threats we have in the tier (Rotom/Rotom-F, Blaziken, Qwilfish, Braviary, Stoutland, Moltres, Kangaskhan) and speed ties with Houndoom, Sawsbuck & Drapion which is a risky 50/50. The only non-scarfers that would be faster (not including scarfers and Sharpedo with speed boost), would be Scyther, Cinccino, Charizard, Typhlosion, Galvantula, Manectric, Alakazam and potentially a few more).
It has two amazing abilities, the first one being Tinted Lens, with its already impressive 116 base special attack, we've seen how good of a wallbreaker it is in UU with Choice Specs, admittedly the only reason it dropped to NU in the first place is because of the weather MUs, Entei/Raikou and Salamence we've been consistently seeing in teams. NU, being a much more fragile tier, has several mons that will struggle to switch into this.
The special walls in the tier are the following:
Lanturn, Altaria, Audino, Cryogonal, Gigalith, Bronzor, Lickilicky, Golbat, Mantine, Milotic
(apologies if I missed a few)
The most common moveset for this set would be - Bug Buzz, Air Slash, Giga Drain, Psychic (U-turn is also a very good option to pivot out and gain that momentum). However, I'll be using Bug Buzz as it's the strongest move for it considering its STAB:
252 SpA Choice Specs Yanmega Bug Buzz vs. 252 HP / 252+ SpD Lanturn: 93-109 (40 - 46.9%) -- 38.7% chance to 2HKO after Stealth Rock
252 SpA Choice Specs Tinted Lens Yanmega Bug Buzz vs. 252 HP / 252+ SpD Altaria: 76-90 (41.7 - 49.4%) -- guaranteed 2HKO after Stealth Rock
252 SpA Choice Specs Yanmega Bug Buzz vs. 252 HP / 252+ SpD Audino: 85-102 (40.4 - 48.5%) -- 67.6% chance to 2HKO after Stealth Rock
252 SpA Choice Specs Yanmega Bug Buzz vs. 252 HP / 252+ SpD Cryogonal: 63-75 (35.5 - 42.3%) -- 84% chance to 2HKO after Stealth Rock
252 SpA Choice Specs Yanmega Bug Buzz vs. 252 HP / 252+ SpD Gigalith in Sand: 64-76 (33.3 - 39.5%) -- guaranteed 3HKO after Stealth Rock
252 SpA Choice Specs Yanmega Bug Buzz vs. 252 HP / 252+ SpD Eviolite Bronzor: 58-69 (35.3 - 42%) -- guaranteed 3HKO after Stealth Rock (once Bronzor rests it's a sitting duck)
252 SpA Choice Specs Yanmega Bug Buzz vs. 252 HP / 252+ SpD Lickilicky: 79-94 (36.4 - 43.3%) -- guaranteed 3HKO after Stealth Rock
252 SpA Choice Specs Tinted Lens Yanmega Bug Buzz vs. 252 HP / 252+ SpD Eviolite Golbat: 30-36 (16.4 - 19.7%) -- 8.7% chance to 4HKO after Stealth Rock
252 SpA Choice Specs Yanmega Psychic vs. 252 HP / 252+ SpD Eviolite Golbat: 82-98 (45 - 53.8%) -- guaranteed 2HKO after Stealth Rock
252 SpA Choice Specs Tinted Lens Yanmega Bug Buzz vs. 252 HP / 252+ SpD Mantine: 60-72 (34.8 - 41.8%) -- 79.3% chance to 2HKO after Stealth Rock
252 SpA Choice Specs Yanmega Bug Buzz vs. 252 HP / 252+ SpD Milotic: 67-79 (33.1 - 39.1%) -- guaranteed 3HKO after Stealth Rock
*Bear in mind that Yanmega can also run other moves which are more effective, for example Giga Drain on Milotic and Psychic for Golbat, although it requires more prediction from the user.
As you can see, although Yanmega has a massive drawback of having a x4 weakness to rocks, the majority of its safest checks are also vulnerable to switching in safely due to their weakness to rocks. The majority of these walls are also passive (e.g. Audino/Bronzor), meaning it's difficult to chip this down without getting rocks on the Yanmega's players field. The best check would be Gigalith, but it has no self-recovery unless you want to play a niche rest set. We've suspect tested and banned Venomoth with a similar ability and sweeping potential, and we also banned Medicham for its huge wallbreaking power for the tier.
We also have AV Eelektross, Dusknoir & Magmortar as potential checks but as you can see they can't safely switch in:
252 SpA Choice Specs Tinted Lens Yanmega Bug Buzz vs. 0 HP / 0 SpD Assault Vest Magmortar: 76-90 (50.6 - 60%) -- guaranteed 2HKO after Stealth Rock
252 SpA Choice Specs Yanmega Bug Buzz vs. 252 HP / 0 SpD Assault Vest Eelektross: 85-102 (44.2 - 53.1%) -- guaranteed 2HKO after Stealth Rock
252 SpA Choice Specs Tinted Lens Yanmega Bug Buzz vs. 252 HP / 0 SpD Assault Vest Dusknoir: 56-66 (36.8 - 43.4%) -- guaranteed 3HKO after Stealth Rock
Now let's focus on its other ability, Speed Boost. Unlike Sharpedo, Yanmega has a very high base speed where using Protect isn't mandatory compared to Sharpedo in case its outsped by faster threats like max speed Rotom, etc. Once set up, Yanmega can pretty much clean the majority of offensive Pokemon except for scarf Cincinno, Houndoom (which is rare) and potentially a few others.
The set for this ability would be - Air Slash, Giga Drain, Bug Buzz, Psychic (swapping Giga Drain for U-Turn/Protect if needed).
This is a Pokemon where players can just brainlessly spam Bug Buzz as long as they lead correctly, pressuring several/majority of the Pokemon in a fragile tier where even Pokemon which were originally great in OU/UU (e.g. Milotic and Gigalith) can feel pressured against it.
Yanmega should be suspect tested at the very minimum.