@Elisa
By far, WhatsApp is the most common means of communication in Europe, and all of our guests prefer it. They use it constantly, and it's quite difficult to miss a message. And it's much faster than the Airbnb app; Instant, rather than minutes or hours. And it's simple to send photos and other documents, links to local attractions or Google maps... etc. Also, you can see if the guest has received the message in addition to if they've read it (e g., message received - flight has landed). And WhatsApp doesn't block content that Airbnb doesn't like (e.g., certain links, phone numbers, addresses, etc).
Most communication between host and guest is benign stuff like flight arrival times, any special requests, local info, directions etc. So WhatsApp is much better for this type of communication, and guests overwhelmingly prefer it.
But for anything involving an "issue", it's important to use the Airbnb message system. Just for posterity (not that written proof of guest misbehaviour guarantees that you would prevail in any potential dispute, but it can help).