It would be a nice feature to be able to configure multiple paths for UnityHub to find projects. Many people may have personal and shared locations which they store projects (some may be on portable drives). It would be very handy to have configured search paths for which it could identify projects.
It could potentially check the sub-directories to validate that they are Unity projects (they all seem to have a ProjectSettings folder but that may not be unique to Unity).
Note: I’d suggest by default not to have it recursively search the sub-directories, but have it as an option for those who find it useful.