Victory Road: The traditional Victory Road was hardly present. After completing the story paths, you just wind through a cave on your lizard to defeat 10-something trainers. You approach the Elite 4 building and challenge the Elite Four in the same room. If you are unlucky, the Elite Four battle music gets stuck in a 5 second loop. The dialogue and atmosphere was cringe, unlike the epic Elite Four experience with Cynthia in generation 4, Iris in generation 5 sequel, etc.
Gimmicks: I echo the above thoughts on the more prominent gimmicks (Tera Raids and Paradox Pokémon), but I completely forgot about this third gimmick: Convergent species. They are knock off versions of the Regional forms. I do like most Regional forms because they breathe new live into older Pokémon, for example Sandshrew and Growlithe. I would have been fine accepting Wiglett as a Regional form of Diglett. I have a shiny Wiglett that I like very much. I just don't want it taking up another Pokédex slot.
Anyway, main gimmick, it's just, some of the Paradox forms are superior to the original Pokémon in every way. For example:
Iron Valiant has 590 base stats
Gardevoir has 518 base stats
(Mega Gardevoir has 618 base stats)
Flutter Mane has 570 base states
Mismagius has 495 base stats
They've power crept these Paradox Pokémon, but there is no post-game content to use them at (assuming they won't be banned in battle facilities). Online battles are decent, but many trainers use the same 20-30 Pokémon so battles become stale. Pokémon Emerald and Platinum/HGSS had the Battle Frontier, Black/White had the Battle Subway, Black 2/White 2 had Pokémon World Tournament and Black Tower / White Treehollow, X/Y had Battle Maison, and Sun/Moon had Battle Tree. Galar had some form of the Battle Tower. When are they going to bring back these facilities for offline players to enjoy? For $30 USD maybe?
Lack of post-game content: Besides online battling and running around Area Zero, your options include:
1. Shiny hunt by squinting at the screen to notice subtle shiny color changes (would it kill them to bring back the shiny jingle and then reduce number of spawns - I don't need 36,000 spawn points in Area Zero).
2. Participate in buggy/laggy Tera Raids (good luck if your Pokemon is holding Leftovers or if your menu gets frozen for three turns)
3. Participate in buggy/laggy Union Circles
4. Hatch eggs by standing around a picnic basket
5. Explore an empty region that has occasional frame rate drop (I can't explore the water level at all because of Veluza).
6. Participate in school / Academy tournament
7. Find swords for legendary quartet
8. Rematch rivals because you cannot rematch Elite Four
9. Complete a small Pokédex (I did that 8 months ago)