The docs say these ports can be used for rx/tx com
It is strange that the two pins are advertised as digital pins but are in fact only for serial communication.
if its a lowish impedance circuit it will be a problem.
Exactly the opposite, a high impedance load is no problem but a low impedance load will distort the signals coming in / out to / from the serial / USB chip.
They're high-impedance because electricity can only flow through them one way?