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Sun and Moon has improved 3D and some really solid world building, but they removed way too many existing gameplay elements for me to consider it a step forward. Stuff like the Rotom Dex, Festival Plaza, and SOS Chaining are all less functional than the features they're trying to replace. XY/ORAS wasn't perfect, but the whole 2nd screen interface with the Player Search System, DexNav, Super Training, etc. was a-lot more practically designed.
Ultra has more satisfying content than the originals, but still pales in comparison to a game like BW2, which went the extra mile in bringing us back to Unova after a timeskip. Entirely new cities, routes and dungeons were added to the map. Characters got older and changed in the aftermath of the original game. Post-game was legendary, with lots of additional sidequests, an achievement system, boss battles against every previous Gym Leader and Champion from the 2D era. This was a true swansong, and Ultra feels like a swindle by comparison.
The campaign in both SM and Ultra is also infamously intrusive, frequently stopping you in the middle of the road for tutorials or conversations (not all of which are even important to the plot). Inhibiting your exploration and whatever sense of agency you may have had in an older game.
Let's GO epitomizes much of fans haven't liked about the last several games. It's an entry level experience with lowered difficulty and less features than what we had in the past. At $60 no less.