If you're looking at this product then chances are you have Googled "How to use keyboard and mouse on Nintendo Switch", I have too. This adapter allows you to use KBM on Switch, as well as PS4, PS3 and Xbox One for titles that don't natively support KBM on console, such as DOOM Eternal.
Overall, the adapter does exactly what it says it will, it allows you to use KBM on console rather than the controller, but it does pose its own problems as well as some design choices I would have changed.
Because the adapter is designed to be used on games and platforms that don't use KBM, the game still thinks you are using a controller, something which obviously isn't the fault of Mcbazel, they can't change that. This can make gameplay not as smooth as you would expect if you were playing on PC, or console where KBM is supported. For a game like Fortnite, potentially the game you want to use KBM on while playing Nintendo Switch, this can create a few problems. When playing KBM normally you might press Q to show your floor build and then left mouse to place. But using the adapter, because the game still thinks you're using a controller, you have to go into build mode, then select your build piece. If you've never played KBM then this is something you will likely naturally get used to, but for existing players this can be very confusing to figure out. As I said, this isn't Mcbazel's fault, it just is what it is when you're tricking the system into thinking your KBM is a controller.
But one flaw that I think should be fixed is the changing binds. The adapter doesn't allow you to change your keybinds and comes with a preset list of binds, which don't even really feel that optimized, although for some games they may work better than others. But with a game like Fortnite the binds didn't make any sense. There is a work around where you can manually go into your settings and change your controller binds, which in turn will change your binds on KBM, but they can still only be what the adapter makes you use. For example your edit button may be on the down button on the D-Pad, something you'll never use on a controller but it might make the keybind more comfortable to use. If Mcbazel allowed for customizable keybinds, if its even possible, then it would be a 5 star review.
Saying that, if you want to play KBM on Switch on games such as Fortnite, Warframe, Warface etc, then this is without a doubt your best option. Just understand it won't be quite as smooth as using KBM on PC.