Disadvantages
If the network link connecting primary and standby is over-subscribed, the redo logs are not applied in chronological order, which can result in large gaps appearing in the available redo at the standby. Such a condition results in the standby being behind the primary.
Data Guard restricts use to like platform and to like Oracle RDBMS versions.
Active Data Guard, an added-cost option bundled within "Enterprise Edition" of the version 11 Oracle RDBMS, allows purely read-only access.
Read more about this topic: Oracle Data Guard