missing /dev/ttyACM* on Ubuntu

According to this page on Arduino for Linux there ought to be some devices at /dev/ttyACM*

That is not the case. On a "traditional" Arduino board, like the Uno, you have two chips: the main MCU which you program, and a USB interface chip, which is pre-programmed with code to present a CDC/ACM interface to the computer (which it then creates a /dev/ttyACMx device for).

With the feather you don't have that. You have one single chip. That chip is both the USB interface and the chip that you program - and most importantly, it is your program that provides the CDC/ACM device if you choose. If your program doesn't provide a CDC/ACM device then the computer will never see one, so it will never create /dev/ttyACMx.

In general the Arduino core always adds a CDC/ACM device whether or not you ask for it, since it is often used to initiate a reset into the bootloader. However if your sketch stops working for whatever reason then the CDC/ACM port is no longer there.

The M0 on the feather has a neat trick though - if you reset the board with the reset button twice the startup code recognises that fact and forces it to run the bootloader. That bootloader then presents the needed CDC/ACM device to the computer allowing you to upload new code.

Of course, if your program that you upload then doesn't initialise its own CDC/ACM device, or crashes, the CDC/ACM device will not be presented until you do the double-reset trick again.