Wii and Gamecube running on Wii U are not emulation really, but hardware compatibility with Wii, and gamecube compatibility is just running gamecube games in Wii mode through magicks of nintendon't.
vWii modding is good for Wii homebrew, Virtual console games only on Wii or injects that are only on wii, or whatever. You can still mod vWii and still use injects; though it is not entirely necessary.
Injects have few things going for them; First of all if you use a USB hard drive or whatnot on Wii U for some delicious Wii U games, Wii and Gamecube injects into Wii eShop format are also installed on the same drive. Before you needed to juggle between two hard drives that are not compatible with each other. Wii U would get it's panties twisted for non-Wii U format drive and some backup loaders would get bent from extra drive that they did not ask for - so overall it is pain unless you just got a SD card large enough to host your gamecube and Wii games you want to play, which conventionally is not that many with affordable SD cards.
Also mentioned gamepad emulation; though not compatible with all games, not even all games that are classic controller compatible. Also Nintendon't is compatible with it but analog trigger games require weird button combos, or just using a Wii classic controller (original, with analog triggers), gamecube controller or sony controllers or something.
So; with injects you can install them on your Wii U's USB drive, have your games on your Wii U's main menu and launch them from there, when you exit the game, you return to Wii U main menu as well.
Downsides of injects is that they need Sigpatches, and best way would be CBHC because of rebooting between exiting Wii games back to Wii menu. Not everyone has a DS game readily available or whatever, not everyone can use eShop easily (with Wii U I had to buy cards from gamestop because my Visa Electron would not work with nintendo's eShop until Switch eShop seems to function) or whatever it may be, being comfortable installing CBHC or whatever.
Second thing would be not 100% compatibility. There is few Wii games that just don't want to work even with all kinds of fixes tried, gamepad emulation is not 100% compatible, Gamepad emulation in nintendon't use is not really multiplayer compatible IIRC, and few other things like that, but overall personally I've been loving injection just because they are on same 1tb drive that I use for my Wii U games, and I have awfully lot free space on it anyway.