With the caveat that they haven’t given full details on this yet, I will say that they’ve specifically noted that AC:NH saves can’t be transferred console to console like other data, nor does it support the usual implementation of Cloud Saves through Nintendo Online.
Today’s video stated in small text that they will eventually launch a save recovery process in case of lost or stolen console that may be one time use only. It won’t be in place before launch.
I’m wondering if it’s a technical thing - they tied map generation algos to some unique hardware ID inside the chipset or something equally goofy, making saves fail across different devices - or if it’s all a part of their inexplicably pervasive exploit prevention mechanisms to keep people from, I dunno, duping rare items or whatever.