Howdy, y'all. I'm going to include impressions of the 8Bitdo GBros. Adapter, which we just got in the office this evening, in Ars Technica's eventual coverage of Smash Ultimate. (No, I do not know when that will be.)
For those who've been following the adapter and are curious about its performance: I just played a bunch of NES games with a Wii Classic Controller connected wirelessly, and then the new R-Type Generations release with an old wired GameCube controller. Both held up to my input-lag scrutiny, possibly a hair faster than the Switch Pro Controller. The extra buttons on the top of the adapter double as "home" and "screenshot" buttons, which effectively makes this a superior Switch menu-navigation option than the USB-GC adapter (though, of course, you have the caveat of only one controller per adapter).
Random trivia: The adapter effectively tells the Switch that you're using a Pro Controller, whether you stick something into the Wii side or the GC side. If you plug controllers into both the Wii socket and the GC socket, the Wii socket takes priority. Also, its shape does *not* support the WaveBird adapter's GC plug. The adapter includes instructions for connecting to Windows machines via Bluetooth; I'll test that out later. (Gonna see how nicely it plays with my Android phone as a Bluetooth pad, as well.)
All in all, great first impression. Nice build quality, solid results. Do not take this as a review; I've only tested for roughly an hour of gaming. (The included micro-USB adapter is incredibly short, so I haven't bothered testing it with the Switch dock, if you're wondering.)