You can use the Google assistant on a mobile phone with Whatsapp but not on Google Nest hub's.
Have a look at the link below for more information about calls.
Thanks for providing that information, Cathal_S!
WhatsApp, we’re always trying to improve our user’s experience, but at this moment, linking WhatsApp to your Google Nest Hub Max isn’t available.
We suggest that you send feedback using your devices by saying, "Hey Google, send feedback," or by following the steps on this link. Let me know if I can assist with anything else, otherwise I’ll be locking this thread after 24 hours.