One issue we have in NetworkManager is with net ports that aren't tagged DEVTYPE=wwan in the kernel.  ModemManager knows they belong to the modem because it's using the parent/child relationships.  But NetworkManager only knows about the primary port.  So if eg 'usb0' shows up from a cdc-ether device, ModemManager assigns it to the modem but doesn't expose that until the bearer is created.  But NetworkManager gets the kernel/udev add event for usb0 and since it doesn't have DEVTYPE=wwan, has no idea that it's a WWAN device and attempts to use it like a USB ethernet interface.

Maybe we could add a property to MM to indicate all the ports assigned to the modem, even before a bearer is created, which NM could listen to and then ignore any net device that shows up in that list?

Obviously stuff should get tagged DEVTYPE=wwan in the kernel but that's a losing battle because it's all based on USB VID/PID and new devices appear every day :(