I been testing NES Controllers on the Retro Duo

Super 8/Retro Duo NES to SNES Adaptor compatibility list

incompatible

Doom
Robocop vs Terminator
Donkey Kong Country Series
Mega Man X Series
F-Zero - Could selete your car, But can't go anywhere
Super Street Fighter 2
Street Fighter Alpha 2
Super Game Boy
Super Mario All Stars - Works, But when you try to start a game it will boot you back to the Main Title Screen - It won't matter if you start the game with a SNES Pad then go to the Adapter
Tetris & Dr. Mario - Device Screen pops up
NBA Jam T.E - Device Screen pops up

compatible

Super Mario World - Works Mario is running except waking
Zelda A Link to the Past - Missing the X Button for the Map
Space Invaders - Plug in a SNES Pad to add credits - take it out and plug the adapter in
Super Mario Kart
Street Fighter 2/Street Fighter 2 Turbo - Try to play this on Hard with only 2 buttons