This shift was massive, and while many changes I agree with, there are some that do need to be looked into:
Arceus-Ground: A+ >> S (below Xerneas) - As Aberforth said, SD Groundceus is now much worse and bulky Groundceus has issues with Marshadow or Mega Luke depending on EVs. I honestly don't believe that versatility alone can propel this into S. IMO it's like Abra - the sets themselves are like A or A+ rank but the combined total of them is great. However, when IMO those sets were pretty much all A rank, that shouldn't propel it to S.
Marshadow: Unranked >> A (top of A) - I personally think this little guy belongs in A+, really. I think this has much more of an impact on the tier than Mega Luke, Pogre, Deoxys-A and Ho-oh rn. It has the strongest ghost nuke in a meta where ghost nukes are invaluable, but can also choose to go LO and just sweep. This entire tier update should be a testament to how influential having unresisted STAB, the ability to steal any boost and godly speed and coverage is.
Arceus-Dragon: B >> C (below Mega Kangaskhan) - OK I'd have seen a drop to C+ but C? This still offers a great alternative to Waterceus that can hit Pdon without causing 4MSS, and while Marshadow did hurt it and the SD set is now finally recognized as bad, I don't think this belongs in C. Also it's not below Mega Kanghaskhan as Nayrz said in their post - it's below Landorus-I.
whoops i fixed that - Nayrz
Mega Tyranitar: B- >> B (below Giratina) - Um, why? This thing has most of its special bulk ruined by weather shifts, it faces massive competition from Mega Salamence with a worse typing to boot and Marshadow just rapes this thing with LO CC.
Xurkitree: C+ >> Unranked -
https://media.giphy.com/media/yIsbuPCEOgNHO/giphy.gif
On a final note, I'd like to make a nom of my own:
Mega Kangaskhan to C- (between Grassceus and Blaziken) or even D. This thing's time has come. Marshadow has arrived and it beats the shit out of it with LO CC unless it runs Sucker Punch. If it does so, it either drops Crunch and loses its reliable way of hitting Ghosts or it loses Fake Out which makes it lose its free chip on everything. It also provides no defensive utility vs Marshadow unless you're able to predict Sneak/Spectral Thief/SS7SS. This isn't mentioning that it's still competing with Mega Salamence- which does provide something against non-HP Ice Marshadows -, Mega Gengar - which can trap troublesome threats for Marshadow in return for stacking weaknesses and it can be EVed to survive 1 shadow sneak at full health (max/max hp/def guarantees the survival of 1 hit and yeah while it is a mediocre spread it is something and 0 SpA Shadow Balls still has a 56.3% chance to OHKO) and Mega Luke - which still breaks most teams. The competition is rising while it is falling. Mega Kanghaskhan IMO is not as good as Mewtwo, Landorus-Incarnate or even Kyurem-White, which don't have the same drastic opportunity cost that Mega Kanghaskhan does (They do have opportunity costs but they're not as big), or Iceceus, Steelceus, or Dragonceus, which, while having about as big an opportunity cost, can actually provide huge roles for their teams, with Dragonceus giving an alternative to Waterceus without the 4MSS and Iceceus being able to beat many teams that don't have a Fighting type given Defog support.