Hello Balaji,
Only user tables in user tablespaces can be compressed. Oracle system tables (tables in tablespaces SYSTEM or SYSAUX) can not be compressed. (mentioned in note 1436352 – Oracle Database 11g Advanced Compression for SAP System)
Indexes can't be compressed during SWPM installation.
Set Parameter according to note 1171650 and 1431798.
I have installed several systems with Oracle ACO, I don't see lot of disadvantages (except for some bugs within Oracle, now have been fixed in new patches)
Regards,
Ning Tong