All databases have rules that define what characters can be used in object names. Most allow the creation of object names containing unsupported characters provided the name is delimited with quote marks or brackets.
ArcGIS does not place delimiters around object names when querying the database. Therefore, if the database does not support the characters in an undelimited object name, ArcGIS cannot support them.
For example, even though Oracle allows the creation of a user name that contains a forward slash (/) or dollar sign ($) when created with quotes, ArcGIS queries the database with the unquoted user name, and the connection fails. Similarly, PostgreSQL allows the creation of mixed case object names, such as database names or table names, if the name is delimited when the object is created. However, when ArcGIS queries the database cluster with the undelimited, lower case name, the objects are not found.
Read the appropriate DBMS documentation for information on supported undelimited object names.