Well USUM has been out for almost a month now so here are some thoughts on the new mons and current meta
Ultra Necrozma
This was the addition most people (myself included) were most excited about, and while it's certainly a solid addition Ultra Necrozma hasn't really blown me away so far. It's definitely a powerful addition to the tier, Neuroforce combined with a decent variety of coverage moves can make it hard to check and a 200 base power z-move is no joke either. In my experience Ultra Necrozma's biggest flaw is how reliant it tends to be on 50/50 predictions to work properly compared to other sweepers. While the unique mechanics of Ultra Burst are part of why Ultra Necrozma is so strong, it also puts a lot of pressure on the Necrozma user to get the timing right, bursting too soon can result in Necrozma getting knocked out but delaying Ultra Burst can leave Necrozma without its z-move for a turn which effective ruins its chances at breaking past bulkier threats. Its reliance on Light the Burns the Sky to break past things like Celesteela and Arceus forms also forces 50/50 scenarios in which the Necrozma user must avoid using its z-move into a Protect in the case of Celesteela or into the face of one of the many Dark-types that have become increasingly common in the meta. I've personally found picking coverage moves difficult, currently I've been liking Sunsteel Strike + Stone Edge in addition to Photon Geyser and Swords Dance but I've had many games where I wish I had Brick Break for Ferrothorn and Arceus-Dark or Earthquake for Steels and Primal Groudon. Overall, I think Ultra Necrozma will thrive once people stop overpreparing for it and start using 1 or 2 checks like Celesteela or Yveltal, at which point it can better take advantage of its z-move and coverage.
Dusk Mane Necrozma
Dusk Mane Necrozma has exceeded my expectations, I was sceptical at first due to how 'meh' Solgaleo ended up being, but Dusk Mane Necrozma has really become what Solgaleo wishes it could've been. Offensively Dusk Mane has several options, it's most often used as a base for Ultra Necrozma but can work on its own as well. It's universally accepted at this point that Dusk Mane is the better base for Ultra Necrozma due to its better defensive typing, which allows it to set up on Fairy-types that could threaten it in Ultra form. Besides the obvious Ultra sets, Dusk Mane has some fun options of its own like Dual Dance, Automize + Weakness Policy, Swords Dance + Z-move and Substitute + Swords Dance. Personally I've enjoyed defensive sets the most, which can take on basically every Fairy in the meta thanks to reliable recovery, Stealth Rock is always nice too. Prism Armour helps this thing loads and doesn't get enough recognition for how good it is, for example it lives an Adamant Mega Rayquaza V-create or an Adamant Primal Groudon Precipice Blades from full with max HP investment, on the more practical side it takes less than half from a supereffective Hidden Power from +2 Xerneas and roughly 40% from any coverage move a Calm Mind Arceus can run at +1. Overall I'm really happy with this Pokemon and expect it to have a lot of influence on the meta in the future.
Dawn Wings Necrozma
Basically a worse Lunala due to lower Speed and lack of Shadow Shield. Its only niche is transforming into Ultra Necrozma but even there it's not nearly as good as Dusk Mane. Don't really have much to say about this, it's just not that good.
Yveltal
If there's one clear winner from the current metagame shifts it's Dark-types, Yveltal probably takes the cake for being the best one in the current metagame though. Defensive Yveltal sets are great on balance at the moment as blanket checks to a lot of physical attackers, Defog was a great addition to its support movepool that gives balance teams more flexibility in terms of how they use support Arceus forms. Choice Scarf sets and Life Orb sets are very good too, finding more opportunities to come in thanks to the prevalence of the new Necrozma forms. Whether Yveltal remains a top choice remains to be seen, when Necrozma usage drops down a bit I suspect Yveltal usage will drop as well, similarly to what happened in early SM when SD Arceus-Ghost was being hyped up.
Magearna
While it gained competition as a Xerneas check with the introduction of Dusk Mane Necrozma, Magearna is in a really solid place in the meta right now. Its main niche over Dusk Mane Necrozma is being able to take on the Dark-types that are currently everywhere in addition to Fairies, and given just how prevalent things like Yveltal are at the moment that's very much a niche worth considering. Magearna functions essentially the same as it did pre-USUM, but it did get one notable addition to its movepool in Heal Bell. Heal Bell is great on balance teams that otherwise are annoyed by Toxic Primal Groudon or Toxic Ho-Oh, though it can be difficult to find a spot for it in Magerna's moveset. I've also been experimenting with Z-Heal Bell, though I've found that the ability to only recover once in a match is a huge detriment against opposing balance teams. Overall I think Magearna benefitted from the meta shifts that occurred in USUM despite having new competition as a Xerneas check.
I'd say overall the meta is tending towards offensive teams as it tends to at the start of a generation. When the new toy syndrome wears off for the new threats I think the meta will be fairly diverse, though it's difficult to say exactly what threats will remain on top or which ones will rise. I'm interested to hear what other people think about the current meta and what some of the other winners or losers are after the USUM metagame shifts.
