Thanks for the prompt answer! I will try to lay down all the useful information I can.
First of all, I play my games on the Wii U Gamepad screen. They have usually worked.
I have tested more games, and this is what I have collected:
- some games are still working (such as Fire Emblem Path of Radiance and Def Jam - both if which are GC games)
- some games that used to start, now freeze: for example, Gamecube remake of Bonk Adventure, and Turbografx16 Bink's Revenge.
The game asks if I want to use the Gamepad screen only, I confirm, then the splash screen is visualized, the Wii banner is shown, then black screen. The only way to switch off the console, then, is to detach the power plug from the wall.
I prepare all my GC games with Teconmoon WiiVC injector.
For the Turbografx, N64 and Snes I use UWUVCI AIO.
Thank you for helping me!