From the guest perspective - if a host wants to send automated messages, I appreciate when they do this solely through the Airbnb messenger, so that at least I have all the information available in one place (and also visible to customer support in the event of an issue).
I wouldn't want to receive automated messages through Whatsapp. This is solely for direct personal communication. If a guest has booked multiple Airbnbs during their trip, any automated messaging you move offsite can result in confusion; also, many people who receive a text from an unknown number will instantly ignore or block it as spam.