Primal Groudon is the most versatile and threatening Pokemon in Ubers and is therefore a Pokemon that cannot be ignored when building and playing in the tier.
Great attacking stats and Desolate Land give Primal Groudon crazy offensive prowess both physically and specially, making it difficult to properly handle.
A very good typing combined with solid bulk, a great movepool, and Desolate Land also give it huge defensive utility, even when running offensive sets.
All of the above combine to make Primal Groudon a Pokemon that can be seen as a blanket check to the majority of the metagame, having an option against whatever it desires to beat or otherwise counteract.
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Arceus-Ground is one of the best Arceus formes in the tier, as it is able to efficiently run both offensive and defensive sets, thus making it quite easy to fit on teams.
Offensively, Arceus-Ground has a lot to offer, as Ground-type attacks are only resisted by Bug- and Grass-types, and, although Flying-type Pokemon are immune to Arceus-Ground's attacks, it is able to run Rock-type and Ice-type coverage moves to deal heavy amounts of damage to them.
Arceus-Ground has a lot to offer defensively as well thanks to its 120 / 120 / 120 bulk and access to brilliant utility moves such as Stealth Rock, Defog, Recover, and Toxic. Furthermore, Pokemon that can hit Arceus-Ground with super effective STAB damage are rare, with Primal Kyogre, Arceus-Water, and Ferrothorn being the only common Pokemon in the tier capable of doing so.
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Thanks to its solid bulk and Grass / Steel typing, Ferrothorn can act as a check to Primal Kyogre, the majority of support Arceus formes, and some Xerneas variants.
While Ferrothorn lacks reliable recovery, its immunity to Toxic and resistance to Stealth Rock, combined with Leftovers, Leech Seed and Protect, can make it frustratingly hard to wear down over the match.
It is also one of the very few Pokemon in the metagame that can provide Spikes support, something that not many teams are able to afford but is highly appreciated.
Access to strong STAB move in Power Whip and Gyro Ball keeps Ferrothorn from being too passive against Mega Gengar, Xerneas, and Primal Kyogre with Substitute, meaning it can't easily be turned into setup fodder.
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Necrozma-DM stands as one of the best Pokemon in the tier. Its typing, ability, movepool, and general offensive presence all contribute to nearly unmatched levels of utility and versatility.
While Necrozma-DM's offensive typing isn't great, it has an expansive pool of coverage moves and setup options and access to an immensely powerful Z-Move, Searing Sunraze Smash, all of which narrow its defensive counterplay.
The combination of Necrozma-DM's typing and ability makes it the strongest check to Xerneas and enables it to blanket check the majority of setup sweepers. This allows Necrozma-DM to provide valuable defensive utility to its team regardless of its set.
Necrozma-DM's potential can be expanded even further through Ultra Burst with Ultranecrozium Z. A sudden change in typing and a boost in Speed allow Necrozma-DM to have an upper hand against threats it otherwise cannot beat and overcome its slow Speed after transforming.
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Unfortunately, there are so few people playing gen 8 National Pokedex Ubers that it's not even a challengeable format on Showdown!, so I could find information only for National Pokedex AG.
Necrozma-DM is one of the most useful defensive Pokemon in AG due to its overall good bulk coupled with Prism Armor, wide support movepool, and typing making it the most reliable check to two dangerous threats in Zacian-C and Xerneas, which makes it mandatory on almost any team. Dragon Dance can be used if a team already has a Stealth Rock setter, allowing Necrozma-DM to potentially set up on foes like Zacian-C and opposing Necrozma-DM before devastating and outspeeding foes like defensive Eternatus and Marshadow after a couple of boosts. Sunsteel Strike gives Necrozma-DM the means to KO Xerneas while chipping down Zacian-C. However, if opting for Thunder Wave, Necrozma-DM should run Iron Head in order to paraflinch foes like Yveltal and Eternatus while having more PP. Earthquake consistently deals good damage to Zekrom, opposing Necrozma-DM, and Zacian-C while 2HKOing offensive Eternatus. Earthquake is particularly good alongside Dragon Dance as well, as few resist both it and Sunsteel Strike, and it turns Necrozma-DM into an obnoxiously sturdy Dynamax user between Max Steelspike boosting Defense and Max Quake boosting Special Defense. Thunder Wave is an option over Earthquake to cripple fast foes like Zacian-C, Calyrex-S, and Eternatus, but it makes Necrozma-DM setup fodder against Groudon due to its immunity to Thunder Wave. Alternatively, Knock Off cripples switch-ins to Necrozma-DM such as Groudon, Ho-Oh, Yveltal, and Eternatus for its teammates. Morning Sun is essential in order to keep Necrozma-DM healthy and check the likes of Zacian-C and Xerneas without being heavily chipped. However, use Morning Sun with caution, as it has very low PP.
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