Currently 'public' projects allow public viewing, but not public editing.
In certain cases it would be really useful to edit a project from a team of mappers, without requiring any sort of login.
The ideal workflow would be:
One user downloads the project from QField cloud web via QRCode --> the user shares the QRCode with the mapping team / colleagues --> all users can load the project and edit geometries / attributes freely.
If it's a truly public mapping campaign, such as mapping geometries for OpenStreetMap, I don't see this as an issue. The geoms will be validated prior to any upload to OpenStreetMap.