Some of the disadvantages of using RMAN are:
If the location of the backup set is changed, it is required to
register it using RMAN; otherwise it would result in hanging situations when
trying to restore backup.
There is no relevant method to know whether during recovery,
database restore is going to fail because of missing archive log file.
If tape backup is used, media management is compulsory.
Incremental backups though used to consume less space, are often
slower as they use to read the whole database to find the changed blocks.