These four have strong Flying-type STAB moves available to them, in the form of Drill Peck, which is kinda weak, Brave Bird, which has recoil, and most importantly, Flying Gem Acrobatics, a spammable 110 BP move with no drawbacks. I imagine the four of them will be quite dangerous to face, especially Landorus-T backed by Sand. Landorus now also has access to Spikes, being able to set them up in the face of walls such as Skarmory.
Ferrothorn has would-be reliable recovery in Synthesis, but since it enjoys Rain so much it's probably not a good idea. Besides that, access to Sleep Powder, Horn Leech and Metal Burst make Ferrothorn the defensive threat it's always been.
Important calc: 252 Atk Landorus-Therian Acrobatics (110 BP) vs. 252 HP / 48 Def Ferrothorn: 148-175 (42 - 49.7%) -- guaranteed 3HKO after Leftovers recovery (Ferrothorn can strike back with a powerful Metal Burst, coming close to an OHKO thanks to Iron Barbs)
The Gen 4 Ground-types that start with G all get access to Spikes, which is an improvement over Stealth Rock in most occasions. Garchomp also has access to Dragon Dance, putting it slightly over the edge.
All Psychic-types receive Psystrike, a direct improvement over Psyshock.
Both setters gain strong nukes in Rain-boosted Water Spout and Sun-boosted Eruption. Even after Ninetales is forced to switch into Stealth Rock, it can utilize potent new moves in Searing Shot and Fiery Dance.
How does Rotom work? Are STAB mechanics going to be consistently applied through generations in Rotom's case as it was changed from Gen 7 to Gen 8? If old mechanics are applied, Rotom-Wash and Rotom-Heat can spam Water Spout and Eruption respectively, while the other forms can't do much. If new mechanics apply, Rotom has reliable recovery in Roost (or the unreliable Synthesis in the middle of so much weather) and access to Scald, Searing Shot, Energy Ball, Aeroblast, among other strong moves.
Thundurus-T is an incredibly potent sweeper with access to strong Flying-type STAB. It lacks access to Weather Ball in its native generation, but I imagine it's quite the threatening sweeping force.
With no reliable recovery, Tyranitar is not as great as you'd expect. Newfound access to Sucker Punch and Spikes are good new additions, though.
The pink blobs can carry lots of new moves such as Roar, Whirlwind, Rapid Spin, Transform, Yawn, Glare, Reflect Type and Conversion. These two will always be viable, so I won't bother commenting further.