I know this thread is a bit old, but I can confirm that the Mayflash Genesis / Mega Drive adapter does indeed work with Retropie / UAE4ARM for Amiga emulation. I haven't had a chance to play with it too much but I was able to play Datastorm in simultaneous two player mode with two original Konix Speedking joysticks. I could move around happily and fire my guns - cool! I didn't have time to try out any other games or joysticks, but initial results are good. The Mayflash adapter is way cheaper than any other 9 pin adapter I could find (especially dual) so this was a real win.
Couple of things to mention though...
-Configuring the input confused me for while because I wanted to set the fire button to B. However I discovered that you can't skip button A during setup. I therefore set fire to A and assumed I would need to edit it manually in the config file afterwards. However when I launched the game, the fire button worked just fine, so no need to change. Button settings are below.
-Bizarrely even though I had configured the input, pushing the joystick right in the Emulation Station screen, opens up the EmulationStation menu, as though I have pressed Start/Select. Works fine in the game though. This is not really a problem as I can use the keyboard to navigate or just cycle left though the menu!
-In UAE4ARM you will need load the rp-a500 config and set input Port 1 & Port 2 to the Mayflash and save.
D-pad Up - Button 12
D-pad Down - Button 14
D-pad Left - Button 15
D-pad Right - Button 13
A / Fire - Button 0
(Oddly these mappings differ to what is shown on the Mayflash packaging)