Reporting gives a high-level overview of which nodes succeeded. All known error conditions are caught and error conditions are reported to the database. However, when a process status is in progress, errors can occur or failures can occur on the node that are not caught (for example, the machine went down or the process was killed).
The Distribution-level reports show the view from the Distributor side and are very useful for checking which Subscribers succeeded or failed to receive a particular Distribution. The Subscriber reports are used to determine which Distributions a single Subscriber has received.
Subscribers that did not attempt to receive the Distribution (because they were not set up correctly or were not running) will not have information displayed on the report. You can compare the number expected against the actual numbers and look for missing Subscribers on the report. Once Subscribers are set up and have been functioning, this should not be a common problem.