Why a private road creates two problems at once
An unadopted road is owned by somebody — frequently, in pieces, by each of the houses fronting it. Nobody has granted general rights over it, so passing along it and running services under it both depend on rights your title may not contain.
The service question bites when work is needed. Repairing or replacing a drain or a supply cable means entering land you may have no right to enter, and the moment that becomes urgent is a burst pipe rather than a convenient afternoon.