I think that the difference between the backlash that Let's Go got and what Sword and Shield is getting is the fact that the vocal parts of the fandom that were against it knew that there was a mainline game coming out that would have what they wanted. Well, Sw/Sh is that game, and it's removing some Pokémon and a core mechanic in Mega Evolution that people liked for what seems to be little to nothing in return. Dynamax is divisive at the moment and the animations look little to no better than what they were on the 3DS. And to make matters worse, this game is releasing on a home console; let me reiterate that: A HOME CONSOLE. While I know that eventually there will be too many Pokémon to fit in one game, the 3DS games could hold 800+ Pokémon with only some lag. And this is the OG 3DS, not the New 3DS. If the OG 3DS can handle 800 Pokémon, why can't the Switch, which is how many times more powerful than the 3DS? This just reeks of laziness and taking shortcuts, which is what has people so frustrated. After all, while I'm a fan of Gen 1, and started with Gen 1, I like Pokémon from other generations too, such as Donphan, Sceptile, Infernape, Serperior, Greninja, Aegislash, etcetera. What if they don't get in? Add to that the fact that Mega Evolution, which really helped two of my favorites in Charizard and Mewtwo, isn't in and that's why I canceled my pre-orders for the double pack and put it towards a game I'm getting in the near future. Also note that fans in Japan are angry about this as well, so I think that this is worse than the backlash against Let's Go. This is what fans were waiting for. And they have been sorely disappointed. And worse, depending on which Pokémon can get in, some Pokémon Go players won't be able to send their favorites to Galar, either. And this when other Nintendo franchises are pulling out all the stops with their games, such as with Zelda: Breath of the Wild, Super Mario Odyssey, and Super Smash Bros. Ultimate. Compared to those games, and from what we've seen in upcoming releases like Fire Emblem: Three Houses, Super Mario Maker 2, and Animal Crossing: New Horizons, the shortcuts being taken with Sword and Shield are standing out that much more.
Edit: Now, there are things that can get me to pre-order the game once more, such as patches that would shore up the Galar dex and allow all Pokémon to be imported into the game and be playable, but until then, I'm saving my money. Also, one other thing that just popped into my mind: what if some of the Poké Ball Pokémon, or heck, even some playable Pokémon in Super Smash Bros. Ultimate are not in the Galar dex? How weird would it be to have some Pokémon in Smash that are unusable in Pokémon Sword and Shield?