The whatsapp migration didn’t happen as part of the initial setup, but it was really easy to do by going into the WA settings on iphone whilst the still connected and using their own transfer wizard. Took about 5 minutes.
The one thing I would say about the whole process is that it does take approx an hour or so, so best not to try going anywhere in that time as the phones need to remain connected to wifi too. I moved between two wifi points for a short while and it possibly interfered with the process, but it is hard to say conclusively.