In 2004 IOUG tech session, a guy gave a lecture ... he was joking "RMANmake a man woman...". Anyhow, he did discuss some shortcomings of RMAN in comparing to old hot backup.
This article discussed about lack of enthusiasm from DBA community to use rman
http://otn.oracle.com/oramag/oracle/03-sep/o53rman.html
Top reasons sighted for not using rman:
- I have something working for years, why break it?
- Rman has steep learning curve
- Rman is complicated to setup and manage