The "EXP-00091 Exporting questionable statistics" error happens when you export table data with its related optimizer statistics, and Oracle cannot verify the currency of these statistics.  When CBO statistics are created/updated with dbms_stats, the time is noted, but it’s always a good idea not to export statistics since they should be recalculated after import.

You can remove the EXP-00091 error by exporting with "statistics="none" or by setting the client character set (e.g. export NLS_LANG=AMERICAN_AMERICA.WE8ISO8859P1).

The Oracle docs note:

    EXP-00091 Exporting questionable statistics

    Cause: Export was able to export statistics, but the statistics may not be useable. The statistics are questionable because one or more of the following happened during export:

    - A row error occurred, client character set or NCHARSET does not match with the server, a query clause was specified on export,

    

    - Only certain partitions or subpartitions were exported, or a fatal error occurred while processing a table.

    Action: To export non-questionable statistics, change the client character set or NCHARSET to match the server, export with no query clause, or export complete tables. If desired, import parameters can be supplied so that only

    non-questionable statistics will be imported, and all questionable statistics will be recalculated.

Source : http://linux-oracle-dba.blogspot.com/