Yanmega is by no means a bad pokemon, but it's nowhere near good enough to rise. This was just recently discussed and any recent arguments made against it rising are still relevant. Yanmega's speed boost and tinted lens sets are both pretty neat, but are riddled with flaws that keep it from being put to full use almost ever. Yanmega on your team means you'll need spinning or defog support since it has that nasty rocks weakness, which doesn't seem all to bad until you realize that it has terrible defensive synergy with almost any means of hazard control. Pairing it with literally any spinner or defogger in the tier will stack weaknesses on your team and force you to build around Yanmega and the hazard controller to find switchins to their shared weaknesses. Yanmega is also extremely lacking in terms of defensive utility for its team, as it has no access to decent bulk or status moves (unlike the better choice in Venomoth) and can't even fit status into its movepool anyway. Speaking of Venomoth, it's almost ALWAYS a better option for your team, as it has a better defensive typing, access to sleep powder/stun spore, and essentially both of Yanmega's abilities with tinted lens and quiver dance (which does more than just boost its speed). Yanmega also has a lot of bad matchups vs the tier if it's using either of its abilities. Overall, the ability to click protect on Yanmega and outspeed a large portion of the tier is pretty great, but it's terrible bulk and defensive synergy with required teammates and lack of immediate power (2hkoing Conk with stab air slash isn't all that impressive considering it's a fighting type) and a plethora of other flaws that Venomoth does a better job compensating for. Again, it isn't bad, but is perfectly fine where it is, as a 4x weakness to stealth rock and the other above-mentioned flaws should automatically make it as situational as stuff like Zoroark and Slurpuff (which are actually way easier to use and don't require as much support).
I respectfully disagree, Hilomilo. Venomoth doesn't fill the same roll as Yanmega, not that that's the main problem with this response. The typing difference is great against fairies, but you need to stay unprotected to get your boosts, while not being ground immune. Banded Krook eats Venomoth for breakfast as do many of the mons I mentioned like Hydreigon. However, I wasn't suggesting to put Yanmega above Venomoth and certainly acknowledge the 4x rocks weakness after we've lost mence and zapdos. Pretending Zoroark or Slurpuff are less situational is something that I find more of an issue with. Zoroark rarely gets a kill, beats literally 0 of the S rank mons that Yanmega does, and I have never seen it sweep in UU. Slurpuff is old news with the steel spam in response to Sylveon. With the immense speed and power creeps in the tier, it rarely gets a chance to Belly Drum, and when it does, there is usually a steel waiting to revenge it. Yanmega doesn't need to stay unprotected to boost, doesn't need to play mind games with illusion, and doesn't need to sacrifice half of its hp like Slurpuff(providing you have hazard removal). In the current meta, I think it deserves at least C+.