Update: Starter Arduino IDE code here: https://github.com/supaplextor/arduino-esp-sketches/tree/master/esp32/FINGER_FPC1020A - could work for other grove ports talking serial. (m5atom grove etc)
I have an M5stickC and a groove cable to a Finger unit. It usually times out. The first change I did was remove the buttonC code ported from the M5stack examples, and use the header for M5StickC.h instead. I've tried something similar on an atom lite, with the same issues. Cannot add a user, it timesout. No users registered on the chip so far.