I have an 8BitDo SN30 pro and have similar issues. For some reason the Select and Start are remapped to  shoulder buttons in CoinOps Arcade. “Coin” is R2, “Start” is L2. Exit game is standard, pressing both those buttons together. The D pad doesn’t respond, but the left analog pad can be used and the game buttons seen to work as they should.

However, if you connect the controller to your computer with the USB cable the problem is mostly resolved. Connect while in CoinOps, try a game, them unplug the controller. It will retain the settings allowing you to use the Select and Start as you would on an Xbox 360 controller. The D-pad still doesn’t work, but the left analog does, maybe 8BitDo will fix this in the future. The setting are retained even after power cycling. There may be a button setting to do this, but the manual leaves a lot out of how this controller is set up.

I normally use an Xbox 360 controller with CoinOps, but I love my 8BitDo SN30 pro for classic games and it fits in my pocket easily. Almost every game in CoinOps is playable as intended. It’s my favorite overall controller by far, and it works with my Switch!