You can't use a single Joycon to play Breath of the Wild, for example (at least not as well as with the full controller). You need the second joystick to move the camera, all four of the right buttons and left buttons are assigned to different controls, the plus and minus buttons are different pause menus, and there's a use for the left and right triggers as well, which aren't accessible in single-Joycon mode.

Yes, it's great for Mario Kart (before I got the Pro controller, my wife and I were playing with one Joycon each), and similar games (I assume ARMS and 1-2 Switch are built with single-Joycon use in mind) but it's not enough for every single game.