I just got the retron 3, just tested mario RPG and it doesn't work, castlevania 3 neither, the genesis part accepts super street fighter but sounds horrible, sonic 2 and streets of rage sound near perfect if not perfect. The NES part sounds better than the retro duo, will test more later.
Retro Twin, tried the same mario RPG cart that i tested on the retron 3 and works, castlevania 3 is not working, and the sound is more or less the same as the retro duo, will continue testing later.