Everything works one for one far as inputs and outputs go. The difference is the initial state of the relays. CRP didn't use a charge pump as they trust the ESS to behave properly on start up. The UC300 initializes its outputs to active low which turns on the active low relays in the CRP electronics. You can disable the motors and turn off your spindle when you power down. Then power them on after UCCNC is up on your computer and communicating over ethernet with the UC300.
I've communicated with CNC Drive about initializing inputs to the proper state on start up and they are considering it. It would be safer during start up to have a charge pump to enable relays. That means the three relays built into the custom break out board inside the electronics would have to be abandoned for safety critical task and moved to an external relay board.
I'm 3D printing an adapter as the hole pattern is not the same for ESS and UC300.
I've been running my machine with UCCNC and it works well for me. They have been responsive with bug fixes and suggestions.
EDIT - forgot to add the disclaimer - you are on your own modifying the electronics. CRP sells the plug and play electronics as a complete kit and doesn't suggest changing anything.