I just got a Sega Master System paddle controller working with the Retro Adapter. It works perfectly but there is a compatibility problem with some MAME games I had not anticipated.
I tried to use it with Arkanoid. That game uses a spinner control which can rotate endlessly clockwise or anti-clockwise, unlike a paddle which has a set range it can cover (typically 360 degrees). MAME does not support absolute positioning for spinner controls (or dials as it calls them). The Sega controller reports back a position from 0 to 255 but MAME needs left/right pulses as you would get from a decoded optical Gray code sensor of the type commonly used in mice.
Apparently absolute positions are usable with mice so I will try to set the controller up in that mode tomorrow.
This kind of problem seems to be common with emulators.