Really don't see how Greninja is balanced rn, even though Mawile forces it to run Hydro Pump or HP Fire, and it gained a switchin in Toxapex (which loses to ESensory btw) and faster checks in Tapu Koko & Pheromosa, it's still completely insane to deal with and can adapt its movepool to break past pretty much any of its checks. Beack in ORAS it was banned too fast for people to realise how versatile it is, Choice Band and Choice Specs are actually legitimate options on it. The only reason it seems almost not broken now is because, early on, people are either going for extremely offensive or extremely defensive teams since those tend to be pretty straightforward. Stall always handled Greninja well, and while HO teams don't exactly like Greninja, they can at least prevent it from coming in, and revenge kill it if it does. But once the dust settles and people want to run bulky offense or balance, and then realise they can't b/c Greninja anihilates those playstyles completely, I really, really don't see it staying.
Mawile is interesting, I don't think it's as good as Greninja, partly becuase the Sucker Punch nerf hits it relatively hard (no longer has a chance to OHKO Heatran at +2 atfer SR damage for instance), partly becuase it can't threaten bulky teams as much becuase they all run Toxapex, and partly because of Tapu Koko revenge killing it for offense while also being a ridiculously good Pokemon. I still don't think it'll stay OU though, becuase it kinda has the same effect as Aegi has, aka making a bunch of key Pokemon way less viable b/c, if you do carry them, you need to also carry a Mawile switchin, which can lead to problems down the line by making your team too passive etc. Moreover, I don't really see the value of keeping it in the tier. Any defensive utility it offers (checking Weavile, Latis, Lele, Zam, non Heat Wave Torn-Ts, etc) is already offered by Magearna, so it doesn't really help the tier in any way. Come think of it, the same applies to Genesect, it use to be pretty nice to check Fairies and Latis while pivoting, but that's no longer as needed. Same for Aegi even, I guess it actually beats Pherosoma unlike Magearna, but Pherosoma is fucking broken anyway so w/e. Aegi doesn't look balanced in the slightest, by the way, especially since its former main check, Bisharp, is a lot worse now, and its other big check, Mandibuzz, is nowhere to be seen due to the prevalence of Fairies.
On the flipside, Landorus seems perfectly fine in this metagame. Not for any perticular reason, it just doesn't perform particularly well since balance is nowhere to be found. Hoopa-U is a bit harder to handle, but if you're like most people, you're using HO and can just steamroll the poor thing. Maybe they'll be banned eventually, maybe not.
Also, can we talk about Dugtrio? +20 in Attack is a surprisingly big difference for it, it really expands the amount of stuff it can trap. Heck, it 2HKOes Calm Clefable with Choice Band, which is kinda insane tbh. This might be due to the GeneTrio hype making it a really attractive Pokemon, but it's starting to become really annoying to deal with.
Another thing that was balanced in gen 6 but seems potentially busted now is Manaphy. Z moves may have put this thing over the top. Waternium-Z with Rain Dance lets it choose whether it wants to ruin Offense with Rain Dance giving it a Speed boost, or destroy Stall by Koing Chansey with +6 Hydro Vortex. It can also run Icium-Z, and KO Ferrothorn, Venusaur, Av tangrowth and Kyurem-B at +3 with Subzero Slammer. This thing is ridiculous, especially now that you have to run passive mons like Toxapex and Hippowdon in order to beat the other broken stuff that's in the tier, and Manaphy just mercilessly slaughters most of these. Might be a product of the current lopsided metagame though. We'll see.
This is kinda unreleated to all this, but am I the only one having trouble picking megas this gen? Maybe it's just me really wanting to try new stuff, but I often have to kind of force myself to run one, and even have a bunch of teams without one. A lot of new non-megas compete directly with megas, and are in some ways even better than said megas even when taking mega stones into account. Mega opportunity cost just seems a bit lower this gen.
also lol why is swagger legal