I’ve found an issue an issue with the serial interface in the Arduino core for the Adafruit CLUE and Adafruit ItsyBitsy nRF52840 which means that sometimes there seems to be a delay before serial output appears. Until this is fixed I cannot really recommend these boards for uLisp. For example, when typing commands into the Serial Monitor, often your response doesn’t appear.
I’ve reported the issue to Adafruit here: CLUE Serial seems not to work correctly.
I’d be interested to know if anyone else has encountered this issue. It may only affect the Mac version of the Arduino core as I haven’t tested it on Windows or Linux.