Store EVERYTHING in ASM (datafiles, ocr, voting, redo, undo)
Use ACFS for datapump(expdp,impdp) DIRECTORIES, archivelogs and RMAN backups. This allows any node in the cluster to do the backups and then sweep these online backups to tape after the fact. This is supported as of 188.8.131.52.
1) This Oracle RAC SIG discussion might give you more data points for considering ACFS: http://lnkd.in/PD_RAb
2) 184.108.40.206 ACFS allows for storing backup sets for example. Now it is only a small step to support DB files on ACFS. ;-)
3) Every cluster file system requires a cluster underneath. The files for the cluster (clusterware files) cannot be stored on it.
3.1) Even for ASM, the Voting Files would be managed by ASM, but are accessed directly.