"You can't play 3DS games on Switch, there's only one screen"
"You can't play Wii U games on Switch, there's only one screen"
"There should be Wii VC on Switch"
People don't seem to realize that the Wii U has DS VC, with a screen mode that allows both screens to display on 1 (TV or GamePad). Things like DraStic (DS emulator for Android phones) also allow this. Two screens on one is not impossible….. but there probably won't be 3DS support, VC or not. It's already been confirmed that physical 3DS games will not be supported, and honestly I don't see them bringing in digital compatibility. Regardless of if you think the 3DS is dead/dying or not, it's still too recent and without enough demand to get re-releases.
I explained the screen thing above.. but you still won't be able to play Wii U games on Switch. You see, the Gamecube, Wii, and Wii U all use a specific type of CPU (processor) called PowerPC. The Switch (as well as the 3DS, for the record) uses a type of CPU called ARM. These two types are incompatible, meaning that there cannot be a simple backwards compatibility mode like Wii Mode on Wii U. As for emulation, that's a different thing entirely. Wii Mode or the Wii's backwards compatibility with GC were just sandboxed (limited) versions of the normal system that only used the same amount of hardware the old systems did. This was possible since the processors were so similar the software could be cross-run, it's just like running a virtual machine on your PC. Emulation is using software code to recreate a system, which can be done regardless of processor type but requires an extremely high amount of processing and graphical power compared to a backwards compatibility mode. The Switch (probably) will be able to emulate GameCube, since the Switch's tech is good enough to emulate the older tech in the GameCube, but the more powerful Wii and Wii U tech just can't be handled by it. The issue isn't the screen- it's technical limitations in terms of power. In addition, on the topic of Wii U, they have no reason to make a VC/backwards compatibility, since they're just going to port/remake/sequel the biggest games on it anyways.
Discussion started at here by GamoTron21