I have no doubt the game is playable as the OP says, but that doesn't mean framerate drops and stutters and tons of graphical glitches don't worsen the experience. The reviews I've seen say the gameplay is great but the glitches and performance issues hurt the overall playability quite a bit.
The game is certainly playable.
But games are a "package" that are the sum of it's parts, when you have large, glaring technical issues, it does impact the playability of a title.
Pokemon games are not known for their great graphics, the Switch games have all looked like serviceable modern games but GameFreak has clearly never tried to wow anyone with their graphics. Given how much money Pokemon makes it doesn't really make sense that the graphics are just fine and not great, and this causes some people to make absurd claims like the Switch games look like 3DS or Wii lol. But that's not even the issue here. V/S has major framerate issues and graphical glitches. That is completely unacceptable for ANY game, and even more so for a AAA game, and ever more so for a franchise as big as Pokemon, especially a mainline game.
When a company can sell 10-20 million units per title, then there really is no excuse to invest in the technology to drive cutting edge experiences... Took years for Call of Duty to start investing in it's engine, mostly because of competition from Battlefield and Frostbite taking away potential customers.
Companies understand money.
As for the 3DS comparisons, Sword and Shield had some glaring visual issues that definitely looked like it belonged on the New 3DS XL, I.E. The 2D sprite backdrops being muddy was a prime example, some aspects of the game was a step up with what we saw before however, but there was definitely a degree of inconsistency which shouldn't exist.
But the other issue is that the Switch is able to be docked on a large 75-85" display.. And with it, you do see where the visuals fall short far more readily... And so it's easier for people to pick apart.
I think if the Switch was a restricted to it's tiny display, a lot of the graphics complaints would probably go away.